There's no such thing as "Lemmy together", Lemmy servers are individual instances of "Lemmy software" created and managed by different individuals that are totally independent from each other.

It's like individual subreddits, managed by different mods, each one with it's own rules and beliefs.

Weren't some subreddits split and hated each other? YES

Did that make reddit not valuable as a platform? NO because there were still many subs with amazing people and quality content.

When you have millions of people, divisions are inevitable, it will surely happen here if Lemmy gets enough traction, but I don't see it as a problem, reddit was fine regardless of it, Lemmy will be fine as well.