kbin.social shows up in the instances list (https://discuss.tchncs.de/instances), but I’m having trouble with !nfl@kbin.social. I couldn’t get it to show up in search until I searched for the full URL (https://kbin.social/m/nfl). Once it did, I can join, but there’s no content. See here: https://discuss.tchncs.de/c/NFL@kbin.social
Edit: actually, one post has now shown up, but not the rest.
The behavior regarding search is to be expected: the community is only fetched after being searched like you did.
To the second point: I have read conflicting information on this. Some people wrote that only posts younger than the time the community was fetched will show up. Others have claimed that it’s the last 20 or so posts but that would be contrary to your experience. Maybe the servers just haven’t caught up yet.
It’s notably different from how it has worked with Lemmy instances. With Lemmy I can search by community name e.g. !nfl@kbin.social rather than the URL, and once I’ve done that it automatically pulls at least some recent content. It did this when I joined !knitting@lemmy.world for instance.