True, it's a rule of thumb. But I think it holds in the overwhelming majority of cases -- even if the reviewer themselves honestly don't think so.
> Let’s say they are using Amazon affiliates, if they consistently use those across the site regardless of if they reviewed the product as good or bad, then they aren’t inherently being salesman.
I don't think that rules them out of acting as salesmen at all. Let's say I run a review site with the intention of making money from affiliate links. Mixing a couple of bad reviews in with the good reviews is an excellent tactic for increasing the trust my readers have in the good review, increasing the chances that they'll buy one of those through me.
In my view, if someone is making money by reviewing things, then it's far more likely than not that their reviews will be different because of that.