That’s only true in theory, and if you are actually capable of doing that.
The reality is that most software was already barely working when it was written, it’s poorly documented and if you try to work on it without any help you might as well write it on your own from scratch.
You will also encounter incompatibilities, missing dependencies, etc.
Don’t get me wrong, I love FOSS, I know all the advantages and it’s definitely better than the alternative. But it’s also not a silver bullet. Though this case is pretty cut and dry.