They are redirecting to joinmastodon because mastodon.social is full up, basically.
Mastodon.social happens to be owned by the same people who maintain the server list, but that's not the reason they are redirecting to the list. Other servers do the same thing if they get full up.
@feditips Thanks again! I'm sorry, I think I replied hastily to a message which you then updated with a fuller explanation.
A general-purpose Mastodon server with a 1000 character limit.