So, the real thing that's happening here is:

* Google drop "real" Android source-code updates to OEMs _and_ the public every half.

* But they ship four Pixel updates, including documentation + SDKs.

* Now they added new APIs in a Pixel-only update.

* Google also drop security update backports to "trusted" OEMs monthly (which GrapheneOS have had access to for years).

So, there are now Pixel-exclusive app features on the Pixel SDK version which isn't available to OEMs - but, it's highly unlikely any app developer would actually depend on these new APIs, since Pixel marketshare is tiny to begin with. This in essence just makes Pixels a weird beta-testing device for what will come out a quarter later to "normal" devices, which is sort of an odd business decision, but also a weird thing to get really mad about, in my opinion (I do see what GrapheneOS are trying to do, with having OEMs saber-rattle about not getting features on the same cadence as Pixels, it just doesn't resonate very loudly for me).

However, the API headline seems to bury a deeper lede; in the thread, GrapheneOS also claim that the quarterly Pixel releases contain security content which is not appearing in the monthly backports. This is quite bad and very sloppy if true, since the Pixel releases can easily be patch-diffed and exploits backed out of them. I'd be interested in seeing this enumerated in more depth.