That 2014 opinion has a whole section explaining why the interoperability argument doesn't work here. I also summarized it a bit elsewhere in this thread.

To add more to that, the opinion spends a while talking about why the precedent in the ruling in favor of Activition over making unlicensed games for Sega consoles doesn't apply here, and that's helpful for understanding why arguments about interoperability aren't going well over in the courts. In short, the Court recognized a clear distinction between making things to run on someone else's platform, and making your own platform that is designed to be incredibly cosmetically similar to someone else's, but not actually interoperable with it.

Incidentally, I'm looking to that detail for hope. Most my anxiety around this case concerns its potential implications for the legal standing of projects like WINE. But those projects are unambiguously good faith efforts at interoperability. If the Court ultimately rules against Google, but bases that ruling on the opinion that Google's claims about interoperability were smoke and mirrors, then that would seem to imply that they are specifically ruling against what Google did, and not what open source projects are typically doing.

What you should really worry about is who owns the C language API (Nokia Bell Labs probably???)

You see, in the US, derivative works are a thing. All languages that derive their APIs and perhaps even syntax from C would be derivatives. There's interesting legal arguments to be had there if APIs are ruled to be copyrightable....

Compatibility seems more like an fair use argument and has been argued in the past for hardware in order to allow fair use access to copyrighted material. There is however a distinction between being compatible and being market substitute for the original work, with the later being much more problematic from a copyright perspective.