That says a proxy could use a landing page to do that, not that it has to do it that way.

The underlying requirement of providing source to users is firm, though. How else will a reverse proxy prominently make a statement to networked users?

> prominently offer all users interacting with it remotely through a computer network [...] an opportunity to receive the Corresponding Source

https://www.gnu.org/licenses/agpl-3.0.html

Yes, correct. If this is their best shot at a solution, it doesn't bode well for their interpretation of the AGPL allowing any UX-friendly version of this, let alone a normally functioning API Gateway.