They might have started at the same time.
I cannot even find the source... It's said it was "soon to be FLOSSed" 3y ago:
https://www.reddit.com/r/opensource/comments/e4en47/wtsocial...
3 years ago is about the exact time lemmy was first announced. An early alpha of a rust + react new reddit clone with some weird custom websockets is probably the opposite of how they would have wanted to start their codebase.
https://github.com/LemmyNet/lemmy