There's still some "wider" culture here. Specially in this topic (accessibility), given that at least some redditfugees left as Reddit Inc. was showing a middle finger to its blind users. And from further checking, alt text for images is already implemented:

If the pic above doesn't load, it'll show "The Lemmy logo" instead. But in no moment the interface tells you "hey, add alt text" (there's a feature request for that though). It doesn't show on mouseover either, as in Mastodon, and I think that this is important (it shows non-blind users that the alt text does something).

As such there's still a good chance that this spreads across Lemmy, if implemented better.

I do agree however that Lemmy is more focused on instance culture than the platform-wide culture. That's visible for me as I've noticed that, usually, users behaving too "Reddity" tend to cluster on certain instances, and avoid others. That sounds like a compromise between large scale and seeking what the link calls "the dense, interconnected pattern that drives group conversation and collaboration" - let the kids use the platform, but somewhere that it won't hamper adult discussion.

Though I’m still curious about how distinct of a culture Lemmy had that was distinct from the culture on an instance (with the corresponding “the only way to categorically prevent the culture of another instance from spreading to another was to defederate”).

The relatively higher barrier of entry of the platform as a whole selects people who are a bit more prone to discuss tech, in detriment to other subjects. And even considering your typical user in "reddity instances", he might look dumb in comparison with the rest of lemmy, but he's still an IQ 9001 in comparison with your typical redditor.

(I'm still reading the .pdf, saved it here. Thanks for the link, it looks interesting. As of yet I've focused mostly on the part that you mentioned to be relevant for this discussion.)