How would making a byte-compatible protobuf by simply looking at the existing one be different than say, creating metric-compatible fonts, or, for example, pret

How would making a byte-compatible protobuf by simply looking at the existing one be different than say, creating metric-compatible fonts, or, for example, pretty much exactly what Google did in the Oracle lawsuit? (Copy them verbatim because they shouldn’t be copyrightable...) Even if you subscribe to the belief that the protobufs are eligible for protection under copyright law, which is fair since it seems to be the current understanding even if it is relatively new, I still don’t understand how creating compatible specs without reversing is illicit. It’s fair use, no? I thought interoperability was a valid claim for fair use.