Garwood,
my opinion about how Amazon differs from Xometry is that a buyer on Amazon doesn't ask for something to be made, he goes there to purchase things that are already made - which implies that the manufacturer has optimized their process to increase their profit margin as much as they can
Xometry on the other hand receives the order from the customer, calculate their price, and then look for someone to make it for as little as possible, and as I understand from the other topic - there is little chance of negotiating the price, you either accept or you don't
so if you make something for Amazon, you can plan your production to minimize the cost and sell at your price to the middle man, but in Xometry case - the middle man is dictating how much you'll profit from taking the job, the bigger they'll get, the more data they'll have and the less they'll offer to the manufacturers to get it done to the point where they'll start to see slow down and then adjust accordingly to still have someone accepting the jobs on regular basis
that is the difference as I see it, from a buyers perspective - there is little difference, yes, but the production side of things is quite different