Though I'm sure that's true, a simple thermostat can be built without any electronic components at all, using only the simplest of analog mechanisms, so it's hard to see how it's relevant.

Well, these days they are actually often connected to the internet.

That on its own requires prngs.

I think the point was that if they're fast enough to connect to the internet they will be fast enough to calculate PRNGs. If they're not connected to the internet, they don't need them.

So it needs a PRNG, but not a cryptographically secure one