I would fully expect Linux content on any community dedicated to technology (i.e. programmerhumor); the rest is totally understandable. Though, I have to agree with @CarbonIceDragon, I really don't see as much Linux content as you seem to - granted I use kbin, not lemmy.
I've read that Lemmy is a bit more personally curated than kbin, is it possible you've just accidentally built yourself a Linux bubble?
I won't lie. I mostly don't engage with content I see here. I didn't do that when I was on Reddit either and mostly for the same reason: I don't really have much to say and, even when I do have an opinion, I don't usually want to engage in what's often a protracted debate about something that will probably just end up being frustrating.
That's not to say I haven't had positive experiences on the Fediverse - I've had more here than anywhere else - I'm just not particularly motivated most of the time.
This is the expected behavior on Reddit when you delete your account. None of your posts go anywhere. You have to manually, before you delete your account, edit them to remove their contents. Requests for deletion under GDPR may function differently.