If there are any significant similarities in implementation, independent reinvention seems more likely. The list of similarities appears to me to be mostly surface-level trivialities of marketing copy rather than technical details, and I see no outright plagiarism.

I looked it up because I didn't see a link, and I was not impressed by his original posting to NANOG: https://mailman.nanog.org/pipermail/nanog/2019-February/0996...

Cynically, it not plausible that a big company like Apple could deliberately steal and launch a security-sensitive marquee product in only four months based on a whitepaper. If this author sues, discovery will almost certainly show development started on "Sign in with Apple" at least months before his NANOG message with briefs and requisitions for headcounts and such going back months more.