So you said the IPs are residential IP, but their reverse DNS points to a amazonbot domain? Does that even make sense?

Reverse DNS doesn't mean much, they can set it to anything; can you forward match them to any amazon domain?

It's forward confirming reverse DNS. I assumed that everyone does that by default.

What everyone does by default doesn't matter really here, it's that an IP owner/user can literally set the reverse to any arbitrary domain regardless if the actual domain has a record for that IP. What matters is both match, thats all I meant

xena said "forward confirming reverse" twice which means rdns and then resolving that forward to confirm it matches.

I don't know if it was edited or I missed it in the first post but you're right.

I'd still be surprised if an Amazon domain resolved to a residential IP