With the set top box, the fact that there’s a computer in there is just an implementation detail.
Technically there’s a real computer in my refrigerator, my printer, and my headphones but I don’t have any expectation that these are general purpose computers that are available for me to compute on. I think it’s those expectations that matter and it isn’t always black and white.
I look at my phone as being similar to a PS5. I understood that both devices were somewhat locked down when I bought and in fact that’s part of the reason I bought them. If the PS5 had an authorized mode that could run any software it would be a worse device. If I wanted something more open, those devices are available and that’s what I would have bought (and have bought in the past).