AWS provides services and APIs to support that specific usage. This service is MITM'ing another company's API. I'm pretty sure AWS wouldn't tolerate that sort of thing.

One could think about it as consulting, but productized, on top of a service. Something like AWS is mind numbingly complicated. Amazon has solutions engineers but there's also a big 3rd party market. And some part of that market doesn't need fully bespoke solutions but can be well served by a SaaS that sits on top (of course entailing risk, but it's all risky).

OpenAI isn't there yet, personally I'm bearish on generative AI and openAIs dominance, but they're the major player, and there's a version of the future where they dominate the landscape with a complex enterprise offering like AWS that leaves room for intermediaries. So it's not a bad idea to include bets on people who are building these intermediary products as part of a vc portfolio.

(my understanding might in incorrect but) is it not how cloudflare prevents DDOS attack?

AWS is a whole other beast than the few dozen REST and SSE endpoints at OpenAI.