I think one issue is that we expect to rely on a standard here, while only a proprietary solution, specific to the infrastucture seems to make sense.

Basically the first device in the infrastucture, whether it is a load balancer, a cache, etc. should set (or overwrite, if it's been spoofed) a custom proprietary header with the client IP. And that's what you can rely on - either it's set and you have the IP or its not and you can't really trust anything else.

He mentions Azure and a few others doing this and that sounds like the only correct approach.

(Although there's still an issue when the infrastructure is changed, and the application code is not updated - in that case the old header might no longer be set, and could now be spoofed)