The recent posts are mostly test posts, about people coming from Threads/Nostr/Mostr, someone testing mp3/mp4, someone testing animated GIF, someone saying translate (from English) doesn't work, "hello world" and "good morning" posts, etc. It's mostly the kind of posts you see on newly opened forums, as opposed to the news and random content on long established forums.

Maybe someone mentioned that sign ups are available in one of those other social networks to a mostly Japanese audience, or maybe time zone just worked out better (it's currently 10am in Japan).

Mastodon is really big in Japan in general. Last time I looked it up I think three of the largest five instances were Japanese.

According to this post there's just been a viral Japanese tweet mentioning it, I think: https://mozilla.social/@ginpei/111146848437550978

But you can't get an account here, you can only join a waitlist. So what good is this for anyone?

An opportunity for better user experiences, to not require users to learn and do so much.