You can see the remote community once you search and you can see all posts in that community, from your server.
On Mastodon, even after you subscribe, you CANNOT see old posts on your instance, unless you put each post url individually in the search bar.
So, not the same at all.
As much as I love and follow matrix closely, I can’t fully trust developers who aren’t capable of deploying SSO in their product (look at dendrite mess). Unfortunately, following their SSO ticket chain was a mess and disappointment.