Yes, a lot hinges on how little you can get away with implementing for your use case. If you have an XML config file with 3 settings in it, you probably won't need to implement handling of external entities the way a full XML parsing library would, which will close off an entire class of attendant vulnerabilities.

> Also, even seemingly trivial libraries can have bugs. The infamous leftpad library didn't handle certain edge doses properly.

This isn't really an argument in favour of having the average programmer reimplement stuff, though. For it to be, you'd have to argue that the leftpad author was unusually sloppy. That may be true in this specific case, but in general, I'm not persuaded that the average OSS author is worse than the average programmer overall. IMHO, contributing your work to an OSS ecosystem is already a mild signal of competence.

On the wider topic of reimplementation: Recently there was an article here about how the latest Ubuntu includes a bunch of coreutils binaries that have been rewritten in Rust. It turns out that, while this presumably reduced the number of memory corruption bugs (there was still one, somehow; I didn't dig into it), it introduced a bunch of new vulnerabilities, mostly caused by creating race conditions between checking a filesystem path and using the path for something.