fiatjaf on Nostr: I don't know, but I can't imagine that Instagram or Tiktok have "global search", i.e. ...

I don't know, but I can't imagine that Instagram or Tiktok have "global search", i.e. I would be surprised if it was possible to find a post from someone completely outside of your social circle or whatever they decide is in your interest from 6 months ago given a fulltext query.

And even if it is possible I doubt there is a significant portion of users doing this there.

If that is your definition of "global search" then that is not a crucial feature of social networks so it's not really a problem we have to solve.

Also, if you believe that it is possible to do millions of indexers that index small sections of the internet, why don't you think that approach is sufficient for a thing like Nostr?