Well yeah there are obviously going to be exceptions...and no one suggested that arts students can only do arts. Arts are as hard as sciences, English Lit/History/RS are are hard as Biology/Chemistry/Physics etc. If a clever person decided at the end of GCSE that they wanted some variety, they could probably manage doing bits of both.
What I said was that the average (anyone with a concept of statistics realises that there will be exceptions) student doing Drama/Media Studies could not manage at that same end-of-gcse point to do Further Maths. I don't think the reverse can be said for someone doing Further Maths, if they had decided to do a subject like Media Studies, they would have probably managed to do it, because they're intelligent.