I'm pretty sure Lemmy has been designed specifically so it can't me monetized. If you try to place ads people can just switch to another instance. If you try to split off from the fediverse I'm pretty sure there's enough data on other instances in order to clone your server along with its content (and mind that you don't own the copyright for posts made by users).
I think it's more than just "not for profit" - there was actual effort to make this platform as difficult to monetize as possible (probably as a lesson learned from Reddit lol). Let's begin with the code - it's under AGPL, which means you can't set up a public Lemmy instance without making its code public. This prevents you from creating an improved version and keeping it to yourself to gain an advantage over other instances. Second, the fediverse means that it's less likely for a single instance to become so big that it can unfederate itself without consequences, and while you're federated you can't really place ads - people would just view your community on another instance.
Is lemmy.ml broken?
When I try posting a message in a community on that instance it just shows a spinning wheel forever. Anyone managed to post messages there recently?
How are lemmy and other fediverse platforms profitable?
Reddit migrator here (shocking, I know)...