Twitterverse - oh that's interesting!:-)

Irrelevant software - especially if forum-based (Threadiverse), user-based (Twitterverse), and other content (PixelFed/Insta, Loops underneath that, and whatever Friendica is about) were all to end up conjoined into a single "Feed" of whatever mixture proportions the user wants, that would indeed become a true "Fediverse":-).

But until that time... some of them do still seem somewhat separate, though perhaps artificially so.

federation - I made a post yesterday, but due to federation issues it now sits solely on PieFed (viewable from https://piefed.social/c/fediverse@lemmy.world?sort=new&layout=list), and presumably is not available from any other instances as well (e.g. looking at https://discuss.online/c/fediverse@lemmy.world?dataType=Post&sort=New, I do not see it). That's perhaps a fine example of how the various vehicles that we use to access the Fediverse are distinct from the sources of content - although tbh that type of occurrence is nowhere close to being unique wrt PieFed, as I've had similar things happen with StarTrek.website and heard of many such occurrences with the likes of Aussie.Zone and Programming.Dev, etc.

Some instances COULD theoretically hold content - e.g. PieFed has a Local filter where people discuss the specific issues relating to PieFed software, as well as a trollyproblems community, etc. - but in practice, the vast majority of content derives from Lemmy.World, or wherever the particular community (or magazine) is based in. And this too is not unique to PieFed - e.g. my previous instance Discuss Online likewise has a couple of communities (e.g. !linus_tech_tips@discuss.online), but the vast majority of that instance is as a "general purpose" one that mainly pulls in content from elsewhere rather than holding it on its own.

So such "onramp" servers are common across the Fediverse - whether running Mbin, Kbin, Lemmy, PieFed, or one day Sublinks, that's just a property of how the instance admin chooses to do things, and the people who want (or don't want) to make communities there.

And yeah, if Lemmy.World switched from Lemmy to Sublinks, or to PieFed, or even Mbin, then it would cease to be called "Lemmy" (although other servers would still be that, like Discuss Online), though would still fall under the heading "Fediverse", and whatever mid-range term used like "Threadiverse" as well. Although people seem to hate that term and argue whenever it is brought up.

I totally agree though - such software details shouldn't matter, and rather it's the "content" that we want to aim at, however we end up getting there:-).