The point you’re missing is that it isn’t about integrating significantly different platforms like lemmy and mastodon, but rather connecting platforms that are close enough.
Mastodon federates with misskey, firefish, iceshrimp, sharkey, (all extremely featured and very similar to mastodon), writefreely (more for blogging and writing but absolutely connects), some wordpress websites (there’s a plugin), peertube (you can follow channels to see when videos are posted), friendica and who knows how many tiny custom instances.
This allows massive customisation of your experience.
Anyway, interop is more of a happy side-effect of using a common federation protocol