I don't really see the Necrons being involved. Their AI tech isn't really all that great, canopteks are technologically advanced but are very far from intelligent, they mostly rely on necrodermis and phase tech to be much use at all, while simple Tau camera drones are borderline sapient. There's only one canon named Necron into fleshy stuff and Szeras doesn't seem the type to create a species or even control one. The Tau also completely lack any actual warp protections beyond being the spiritual equivalent of dry toast so being an anti-chaos weapon seems unlikely for that reason as well.
It'd be like Fantasy Dwarves having the no wizards rule but not getting their magic resistance stuff, a flaw not a feature.
The Necrons aren't fond of using AI but theirs is canon the best, and probably the first invented in the galaxy (don't see the Old Ones learning programming). In the old 3rd ed canon the C'Tan Void Dragon is either capable of controlling most all machines and AI and is also likely the source of much of humanity's technology invented after the Dark Age of Technology (so all in all it's rather fortunate ironically that the mechanicus have been stifling innovation), though the mechanicus are also possibly stripping away their humanity on the whims of a toaster god. In the new fluff this is more or less the same but we do get one notable canon fluff of one of the Necron tomb worlds' AI glitching out over the millennia, reprogramming all of its Necron guests - assuming command despite having been invented as a caretaker program, attacking other tomb worlds with its necrons to continue conquering and reprogramming with no end limit. I'm not sure how powerful an AI with the computational power of Necron science and the space of a planet is, but for that matter there isn't much detail over exactly how intelligent even their lesser autonomous machines are - the scarabs, spiders, wraiths and other janitor bots. I wouldn't use sapience as a judge for the complexity of Necron AI, especially since in 40k the real challenge is creating AI that doesn't develop its own will - with the newfluff glitched tomb world being one such notable example where a Necron AI fucked up and stopped being subservient. And then of course there's the oddity of the actual Necrons themselves, but to put things into context: The C'Tan in both new and old fluff have conscious thoughts because the Necrontyr built their minds for them; Necrontyr computer science was quite literally godly. How much of that knowledge some Necron Crypteks or the C'Tan still have is pretty unknown, because the Necrons don't like communicating with the species that make good fluff. But from what we do know, Necron AI is immune to chaos, sapience (sans the one notable exception) self-building, repairing, capable of communication, receiving and processing a wide spectrum of information (light, sound, magnetic fields and so on) and is capable of lasting eternity.
So you have a race of people who in their infancy are about to be exterminated by Imperials when their planet is engulfed in a warp storm for thousands of years. When the warp storm ends, what remains of the planet is not a gibbering mess of chaos spawn, but a technologically advancing and emerging power that is untainted by chaos - being a race of young, short-lived scientists that have incredibly weak warp-signatures. They make use of advanced technology and AI, and have very little knowledge or experiences dealing with Chaos. Where they have eradicated growing Chaos incursions, they have done so not realizing that there was more to the incursions than unusual xenos rebellions, with the few exceptions of Tau who haven't gone insane upon learning the truth being Farsight - who is notable for having stumbled upon some technology which messes with time and is anti-demonic.
That to me speaks to the Tau being the product of either interference from Necron Crypteks, C'Tan or an Old One remnant. If there was an Old One remnant left alive when that warp storm hit the Tau homeworld (or indeed, if the Old One remnant caused the warp storm) then it's plausible that that Old One was capable of protecting the Tau homeworld the same way Emps covered Earth. We also know that the Old Ones are capable of and regularly engineered other species to fight for them in the War of Heaven, and from the Krork we also know that the Old Ones were capable of programming an instinctual knowledge of technology into a species. The genetic caste system and the ethereals controls over them would be of no issue to an Old One remnant. It would also reveal that the Old One remnant had changed priorities, from engineering a species capable of fighting the materially powerful but warpfully weak Necrons, to the warptastic chaos.
The Necron theory rests on the Necrons or C'Tan being involved, as we have seen them be involved with humanity previously. The Necrons possess all the technology needed to shield a world in perpetuity from the warp, with the Cadian pylons being the most dramatic example. We also know that they still possess, and that their Crypteks still continue to do, all kinds of experiments and shenanigans with time and the genes of species. Most notable would be the pariahs or human blanks, who were engineered by Necrons as an experiment against the warp. The low-warp signature of Tau and the ubiquitous of their technology harkens to this and the Necrontyr, and I find it more likely that if it was the Necrons, the Tau were experimented upon by a Cryptek and not the Void Dragon. This would also fit right into the Necron agenda, as they have been continuously seeking ways to eliminate the warp's influence over the materium
Could be Tzeentch, but then unless this was some Nth dimensional Vostroyan roulette strip poker chess, it doesn't make much sense to create a race of blueberries that are pretty efficient anti-Chaos weapons.
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