Lemmy is definitely different from how mailing lists work in practice - much faster to get updates on, more ergonomic, etc., but in theory - could SMTP, IMAP, and a bit of ingenuity be used to turn a mailing list into a Lemmy-like interface for comments, links, etc?
I just had the idea of making a mailing list-based ActivityPub host lol, but it would probably be much slower than Lemmy in just about every way because of protocol overhead.
Back in the early internet days email clients included a Newsgroup reader (Usenet). It worked pretty well, but had its problems. It was not nearly as seamless as a modern web based solution and lacked moderation. I used Newsgroups a good amount back then, but it would get unruly at times and then spammers put the final nail in the coffin.
Usenet is still around and can be a good source for downloads, but now you have to subscribe to a server and indexer. Back in the day ISPs provided a Usenet server with your sub. The big guys killed that off along with ISP provided web/ftp space. The way of the future, less for more.
I would have to say the same thing about any incarnation based on group email, it’s likely to be unruly and hard to moderate. Newsgroups did it, but then died out for its failings.