It's funny you mention that because I was thinking the very same thing on the way home: the possibility that it's actually not Mahler. But it sure looks like him.
Looking over this again - carefully; making comparisons - I think it might be our 'faux Mahler'. The guy at 4:00 and 6:37 has a much bigger head of hair than Mahler normally had, but does have a similar chin and hairline. It's hard to tell just how 'big' the hair is that the person at 5:20 has, because his hair fades into the dark color of the chairs. It sure looks like Mahler, but it seems like an unlikely pose of him that somebody would have had to capture. Hard to say.