I guess if the requirement is to multiply two numbers $x, and $y, $x + $y isn't a bug, it's merely incomplete
No. And you know it. That's another "silly extremes" example. It's just an attempt to try and "win an argument", rather than further a technical discussion.
A photo editor that has a red-eye feature that turns all eyes red, has a bug.
But if it doesn't have a sepia filter, it doesn't have a bug, it just doesn't support that feature.
The difference is clear and easily understood.