Yeah, agree with you.

It's also not exactly "QA" in this case, it's more close to just validating things every once in a while...

Especially if the specifications aren't clear and "atomic" enough, a lot of implementation mistakes will slip in.

If a product manager is the one creating those specifications, they're the main ones who should be going around the product verifying if they're correct or if they actually "work" in practice. Developers and QAs can do it too as part of their job, sure, but it won't be as effective unless specs are ultra-clear, which they often aren't.