Oh, I completely forgot about that. You’re right, that 10 point deduction would have caused the grade to fall to exactly 0.

I was trying to be magnanimous, but ultimately I think both professors have the right of it. This student never intended to participate in the class in good faith - the barbed submission was intentionally baiting out a failing grade and negative response from the professor so that it could be used as fuel for the political culture war.