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.
At their core I suppose Usenet, PHP bulletin boards, and Reddit/Lemmy are all just evolutions of the mailing list concept. At each evolutionary step the interface and implementation details change to make larger user bases manageable or to add desirable features.