Its different way but you will learn a lot of new libraries, ways to mutate objects, lists, all sort of data structures and new things really really fast.
Among other differences, leetcode teaches you little about reading large unfamiliar codebases; debugging; organizing large software-engineering projects; working in teams; teasing out actual requirements; making incremental progress; real-world performance (and the tools you need to identify bottlenecks); and most of the libraries and frameworks that are common industry knowledge.
Doing Leetcode doesn't even teach how to build a 10000 LoC project.
For me those exercises are more about developing muscle memory than really learning a language.
OPs idea is good, but I think fails in the same way. I don’t think you’ll get much value out of reading PRs until you have certain familiarity. No amount of PRs will teach you what a monad is, you need to dive deep and conceptually understand the model(at least IMO).