Another problem with measures of decentralization is taking into account the dynamics. That can also be done (in this case fragility analysis).

Yes, most posts are on one relay. The question is what would people do if it fails for some reason. Would they point to Peter's paper and say "haha, he told you so" or would they move on and use things differently, use other relays, mirror content, etc.

Because that's what matters. With Facebook, you can't do that.

I use public relays, but all my notes are backed up and frequently synced to other relays. When people lose their notes, will they perhaps demand a backup/replication service and use more relays? Very probably. And that is what really matters.