The terms-of-use on every lemmy server I've seen would be considered underdeveloped by any lawyer I've ever met. Pragmatically:

• In the absence of a TOU that requires licensing content to participate, content posted directly to a lemmy server would probably get whatever the default treatment is either in the jurisdiction where the post was made or where the server is hosted (or maybe even that depends on the jurisdiction of each in complex ways). In the US that would mean all content is all-rights-reserved by default.
• But the poster/commenter isn't going to try to enforce their rights against lemmy. If they didn't want the content there, they wouldn't have put it there. And if they changed their mind they can delete it. And if they refuse to delete it themselves but contact an admin/mod... probably the admin/mod will just delete it for them.
• If the jurisdiction where the instance is hosted has a safe-harbor framework of some kind (like the US does), that would provide some protection from copyright claims on user-generated content provided the admins followed the requirements to be eligible (which I think most admins do even if they don't know it).
• Images and media hosted elsewhere but hotlinked from Lemmy may have their own TOU's (like imgur or whatever).

Overall, I'd say most of the lemmyverse has underbaked policy frameworks. The de-facto results function ok pragmatically anyway for what lemmy does on its own. Any scraping/reuse of content from Lemmy would have to navigate a very complex, confusing, and ambiguous licensing landscape. Probably 10y from now, if the Lemmyverse continues to grow, TOU's will be more common and more clear about open-licensing content or leaving it all-rights-reserved but giving lemmy a perpetual irrevocable non-exclusive right to distribute whatever you post here (the latter of which is more or less what's implicitly happening today).