Figma is a lot harder to replace than postman and it's primary users are not developers (although they often consume the result of the primary users). I think developers are more fickle than designers as a customer, especially for tools that are not central to the end result.

If I deploy my app and it works it does not really matter if I tested/documented it with postman, it's not central to the app at runtime.

I also think that postman's core product is a lot easier to replicate (they were not even the first to do it) than figma and there are a number of open source projects that do.

When it comes to their API gallery I would expect that any serious API would come with a non-proprietary definition or documentation (in something like OpenAPI or at least just good docs) too.

I don't know, I just have a hard time to see why this product (and by extension the company) is worth 5.6B.

Developers don't make the call. Companies would ask people to create and share a postman collection with API tests instead of checking it into git.

Valuations are crazy these days. But I don't see how this is crazier than other SaaS valuations. Why github when you can run your own server? Why use confluence when you can check in your docs into git?