Greenleaf is pretty massively exaggerating about the extent of defederation, as only a handful ever get defederated regularly, certainly not enough to call it ‘wars’.
As for UX, there’s definitely room for lots of improvements, especially in making it easier to explore another instances local communities from within your own insinstance without explicitly subbing to them all or using lemmyverse.net.
But I don’t think the very concept of different instances is truly a barrier or bad UX, that other user is just giving lazy excuses for not switching away from Reddit.
If that was a legitimate issue, MMO’s (which also often have servers the player needs to choose) wouldn’t have the userbase they do. Nor would Email have taken off.
Even if Lemmy was one big simple centralized server, that user would just come up with another reason they couldn’t switch.
“Oh, it’s too small, my niche communities aren’t there”
“The UI isn’t as nice”
“The mod tools aren’t as good”
Etc.
Different instances that have a unique interest or theme will determine the type and feel of content in your local feed, and can have a tangible community as you recognize names from your instance. That’s the main difference.