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?