Really it doesn’t.

Sure there are some times where having source access to other projects helps, but in most cases it really doesn’t.

Essentially at best it turns “this function takes more time than I would expect” into the same thing with “... I think because of X”.

You may think “I could fix it” but again in most cases any fix will require you knowing why certain trade offs were made. Designing around internal design also means you may end up having overly complex code that continues to exist if the library changes design to improve that case. It may even become slower, or worse if your software is important enough may prevent future improvements that would benefit everyone.

So no, being open source does not magically make all bugs trivially fixable.