I don't think this is really a "lone developer" problem. If you jump into any large-scale application codebase you'll be lost and confused, even if it was developed by hundreds of developers.
Maybe the reason people perceive this as a "lone developer" problem is lone developers usually share their code for free (or for equity), so anyone trying to get up to speed on it won't be paid for their efforts (at least not right away).
Maybe the reason people perceive this as a "lone developer" problem is lone developers usually share their code for free (or for equity), so anyone trying to get up to speed on it won't be paid for their efforts (at least not right away).