When you go to the download page, you're offered the current LTSish (20.04) or the current interim release (21.10).
It's only spelt out 'below the fold' on the Raspberry Pi bit of MATE Welcome program once you've picked one and are running it that you can't update versions on the Pi in the usual way. I can't spot this on the about page of the website or the download page or...
It doesn't matter so much now - with the 22.04 release coming reasonably soon and the support for 20.04 being shorter than usual (to April 2023 on the Pi rather than April 2025 on other platforms) I would be doing a fresh install in a few months anyway. But come the end of the year, it is going to catch people out: it would have done so to me.