Some services go a step further and specifically ignore an 'accept-encoding: identity' header and return gzip instead, because they feel that asking for uncompressed content is broken.
That is what GCP is doing according to TFA.
As I'm reading it, GCP is ignoring accept-encoding as a caching variable. Certain services (stackexchange API calls for one, but I'm pretty sure it's come up elsewhere) just ignore it entirely from the application.