It's most likely a conspiracy he created to cope with thinking of classic as "superior". Pure tribalism, basically.
It could easily have been a misunderstanding about Toady's intentions. Please avoid these sorts of personal attacks. I know you have a strong preference against the old UI, but not everyone does.
The UI has nothing to do with this. If I were more charitable, I'd say maybe RetiredNavyVet sees every version of .50+ as "premium", and 47.05 as "this branch". This is doubtful unless he's completely ignored all development for the past few years, or hasn't looked at the main page since .50 classic released. Regardless, sorry if I don't immediately take the charitable assumption given how some veterans seemingly want Toady to delete the past four years of development and they've immediately burned all trust with him despite the years they've been playing.
Didn't the devs say that the premium version was a "one and done" product?
Nope.
Starting with the last version (v50.05) all future versions will be available both in Classic (free), and Premium (paid). Both these share the same UI and graphics support, but Classic does not come with a default graphics pack. So if you want to play not-text-character-mode in Classic, you have to wait for modders to make a comparable pack.
The bulk of the new UI and graphics support is done though, so keeping future versions up with Premium should take less effort than the initial migration.
Finally, some features big and small (like Adventure mode, and the summarizing health screen) did not make it to v50. They will be added back, but until then players might want to play 0.47.05, the last version before the UI rewrite. It too is arguably "Classic", since it is available for free on the Bay 12 website, but it is not the only "Classic". If people say just "Classic", it is easily construed as the current/latest Classic version, so 0.47(.05) should be referred by its version number.