I've always maintained that modern schooling has been trying to do to maths and sciences as they've done with English and other nebulous subjects where the 'right' answer is completely up to the teacher. This one teacher has done just that in this case on the internet.
I always hated the whole "give marks for showing working out" because it penalises the people who have learnt their numbers well enough to know the answer as soon as looking at the question. I was always losing out top score in maths to girls in my class because I didn't show the steps correctly. One time, when I had had enough, I showed every possible step, only to lose marks for taking too many steps to 'find' the answer!
The teacher in question knew damn well that I could solve most problems at that age in a line or two (or straight away, but that was just asking to be marked down).
My marks shot up the year I had a male maths teacher whose rule was: "if you've got the right answer at the bottom of page, full marks. No questions." If you haven't, then he counts points for showing workings out.
I'm sure every male reading this has found himself in this same position where a simple maths question these days has a never fully achievable 100% because some teacher (female or male) decides the student answering the question needs to be brought down a peg.
In my Year 11 class I was doing so well in one of my maths classes (would get 100% on the weekly tests most weeks) that the male (!) teacher offered my friend and I to skip the class so we could focus on other studies. Not bad for a guy who would average about 80% in primary school and early high school.
Note: my learning style and relative knowledge didn't change, only the attitude of the teacher.