Kinda. You're server will not back-populate old posts from someone you just followed. However, you should get all future posts.
If you find a thread on Lemmy and you want to comment on it in Mastodon, but you can't find it normally, you can copy the URL to the thread or comment, paste it in Mastodon search, and then it should force-load that post. Then you can reply and comment on it as normal.
A lot of people saying this is a Chromium fork, but According to their blog, that's not fully the case.
So it uses Window's default web renderer. It just happens that the default renderer for windows is the same as Chromium's renderer. In this way it does contribute from the chromification of the web like the Chromium forks, but the browser itself isn't a chromium fork. (This is also why the macOS version uses Webkit and not Blink.)