....for sure, a simple test doesn't say it all. But they need selection criterions, like any company. And because caculation skills will be required, they chose to make this a first selection criterion.
....for sure, it is not all black and white, but that's with any application procedure at any company. If I would have known before what I should have said during any interview at any company, they would have hired me.... But point is, that these companies have the luxureous position to be very critical in their selection due to the attractiveness of the business and the large amount of applications. Then, what would you do? Select the best to outperform competitors with. And the best are proven to be people that at least would be able to calculate at a reasonable pace. And a reasonable pace in this market is finishing with 56 correct answers out of 80 in 8 minutes. With a slower pace, your dealing skills could be perfect, but the deal would be gone by the time you realise there even was a deal....!
After this tests (remember, it was just a first selection), out of the remaining part (still enough people) they INDEED are selecting on all the other skills needed and then they pick out the best dealmakers/ moneymakers.
But one should question whether he would like the job if he doesn't like or doesn't have the ability to calculate quickly...