Underlying technology is completely different. One is Swift, one is React. It talks about this on the GitHub.
This is one of those cases where two is better than one, because not everyone can contribute to a Swift app, and multiple projects isn’t splitting effort, it’s bringing on people who wouldn’t otherwise be involved at all.
Plus, probably worth having a few going just because there is so much activity now. Mlem is moving a little slower than I had hoped so while I’d prefer a native Swift iOS app, this one is already a little more functional and it’s the first release.