But why? You can have the same chip for much less. The "toolchain" is just a bit of syntactic sugar around existing languages and tools, but more to the point, nothing stops you from using it with your own hardware.

I'm not doubting you, just sincerely curious.

If they've done 500k units, "based on a respun Uno" almost certainly means "there's a atmega328p on the PCB, a 16MHz crystal, and some points granting access to the serial and reset pins"

ATMega328p, 32768Hz xtal, in a smart radiator valve. See OpenTRV (for the open part of the s/w and h/w) and Radbot for the commercial product.

That's a cool product! I wish I could put something like that on my hydronic (hot water) radiators, but they don't have individual valves.

That design started ~10Y ago. If I was starting again I might pick something different.