I agree with you. The Dom should be encouraging and rewarding obedience and the sub in the 2+2=5 case obeyed the Dom's command. In my view there are some basic principles - laws if you will - you should not play around with to get your jollies. There is no need to, the Dom could quite easily have said "you answered too slow" or anything that did not compromise the integrety of instructions and obedience, the cornerstones of Ds.
I would have caught the sub by handing her a calculator and asking her to calculate 1852279*2 and give me back the calcultator. Then I hold the calculator upside down and when I find the numbers seems to spell the word "ASShOLE" I go ballistic and accuse her of insulting me. Other "magical" numbers that looks like words are:
83507 LOSER
558.618 BIg.ASS
5318008 BOOBIES
Do all of them on her and she's got to give up. I mean one insult you can forgive but four in a row, no way can the Dom stand for that.