> What do you mean it is not a source of bugs?

I think what they mean, and what I also think is that the bug does not come from the existence of uninitialized variables. It comes from the USE of uninitialized variables. Making the variables initialized does not make the bug go away, at most it silences it. Making the program invalid instead (which is what UB fundamentally is) is way more helpful for making programs have less bugs. That the compiler still emits a program is a defect, although an unfixable one.

As to my knowledge C (and derivatives like C++) is the only common language where the question "Is this a program?" has false positives. It is certainly an interesting choice.