Do they at least enforce that Anker products actually came from Anker and not some scammer who's happy to burn my apartment down?

No. It should hopefully be well known by now that Amazon "commingles" all inventory of a given item. You can never be sure if you'll get a well-made counterfeit on the "marketplace". Even if you PICK WHICH SELLER you get whatever's in the bin that the overworked amazon warehouse sub-sub-sub-contracted employee quickly plucks.

for any anker product i can find on amazon, there is only one seller - ankerdirect - selling on FBA. it's difficult to commingle products when they're all coming from the same seller.

Sellers can choose to not co-mingle, but then they have to put a separate UPC code on the item.

Having done that, can they be sure that another seller won't try to horn in on the new UPC code?

is true for big ticket items like apple products and laptops