The function call is not the issue. The issue is that it is a large unfamiliar code base, and the bug, as in this case, would not be an incorrect None somewhere, but will be a mathematical bug somewhere, like an incorrectly written formula. Chasing that is far far more difficult.
Bearing in mind also that the average phd mathematician has at best cursory knowledge of programming and software engineering, enough to get on with mathematics, expecting them to dive into sage and fix things is just unrealistic.
> it's largely trivial compared to the everyday work you do
I think this is also a very unrealistic expectation of users, however skilled they may be as mathematicians.
It's an unrealistic expectation to expect it from every single user, but not for many of them.
These two programs are a prime example of how open source is just as inaccessible as closed source. It's simply too difficult to learn a large complex codebase. People have their own jobs to get on with.