The bird app (Twitter) calls them tweets, so the Mastodon service decided to call them toots. But, they did rename them to "posts" earlier this year:

https://news.ycombinator.com/item?id=26882318

I guess this isn't reflected in joinmastodon.org yet. Each instance can also choose to call posts whatever they want:

https://www.reddit.com/r/mastodon_social/comments/9mhzu8/how...