How effectively I can multi-task depends entirely on the tasks. If one of the tasks is something almost entirely passive like listening to a fiction audiobook or watching a movie I've already seen and the other is some repetitive activity that I could just about do in my sleep like knitting with no pattern, I can do both at once.

I can listen to a podcast or TED talk while working an easy to moderate Sudoku puzzle and watch a movie I've never seen while knitting. I wouldn't attempt two tasks which both required cognitive attention at the same time. I could probably do it but why would I? If a cognitive task is worth doing at all, it's worth doing well and that won't happen unless I do ONLY that task or I add a non-cognitive activity to it.

I wouldn't need to try the test you mention to know I'm not a Supertasker. My vision impairment would probably cause me to fail the test.