Well, this person claims on their website to have a PhD in math from CMU and to have taught the topic at multiple state universities. You could dig some into whether that's true, and then rely on the reputation of the named institutions if you don't know anything about them.

Just like reputation already worked. People have associations with each other. People endorse each other. Endorsements carry some transitive weight (I suppose you could use graph theory to model this). This has nothing to do with AI. Yes, Amazon is full of constantly changing "brands" with no reputation. That is different from a person or company that does have a consistent reputation that you can look up, even if you hadn't heard of them until today. Even Amazon also has e.g. Samsung or Asus if you don't want to buy AHIKJASFDIUHER.