There's always been a bit of a disconnect on their position about machine spirits. The reason servitors have a human brain is because AI is bad in the 40k universe (an idea they shamelessly ripped off of Dune.) You need a human brain in a servitor to make basic calculations and respond to inputs so you don't have to have an AI do it.
Right, but then what about the machine spirit of a cogitator? There can't be a human brain in every console on the ship's bridge.
So there's a human brain in the computer core a starship, that is half brain and half AI, that runs most of the ships functions. Still making sense, kind of.
But what about the machine spirit or war spirit of, say, Space Marine power armor? The brain of a space marine doesn't run the calculations for their power armor, or monitor and control the armor's life support system. There's no chunk of a human brain in the suits power pack (at least not that I've ever read.)
So in that sense, there is AI in the world, running some basic, invisible tasks.
And that's totally setting apart the machine/war spirit of something like a bolter. The boltgun has no electronic or computerized parts (unless you count the machine spirit that manages its targeter.) Yet the Mechanicus believe that every piece of machinery has some part of the spirit of the Omnissiah. So even a bolter has some level of sentience.
There's basically a breakdown of logic between AI and magical mystical spirits which act like AI but aren't. But that's pretty much par for the course for 40k.