Originally, you couldn't see the reply count while looking at a tweet because the replies were just visible below the tweet itself. Then Musk pushed a ton of changes, and now there's still no reply count, but now you can't see those replies if you're not logged in. Really it's just that the sensible evolutionary changes haven't had a chance to catch up with the breakneck changes.

> Maybe it's obvious when it looks too quiet?

This is probably your best signal. The linked tweet currently has 37 retweets, 10 quote-tweets, and 240 likes, so it would be incredibly unusual if there were actually zero replies (barring tweets which have had their replies limited). But to know that, you'd have to know a bit about Twitter norms, and if you know that then you probably know that you can't see replies without being logged in.