> I don't think the main problem in this article is that "," doesn't look like a function because we are told that it is overloaded so we know it's a function.

Well it's a function in one of the code snippets and not in the other. That it can be both is definitely part of the problem.

> You can get those types of problems in other cases too when you overload functions that have a good name, but maybe it's less likely since I guess the comma operator is already defined for all different types but named functions have to be defined by the programmer.

The problem is that it's not just overloaded but overridden. Subtyping is problematic at the best of times, but the subtyping relationship of C++ lvalues and rvalues is particularly insidious since it is completely invisible in the code.