Still confused about this one

I agree. It would be very helpful to have a video explaining this question.

I find the quest stem confusing and unclear.

Ditto. The prompt doesn't seem to be saying anything about the survivability of left-handed people versus right-handed people...

Hi Michael, This is a strange question stem- it is asking us what hypothesis of M's Q could be countering. While it is far from clear, I think we are being asked what Q could be preemptively arguing against. Since Q supports the idea that the handedness disparity comes from old people being disproportionately rightie, we can conclude that Q does not think that righties are more likely to live to old age. This is another possible interpretation of the evidence that M provides, and Q's additional evidence undermines that point of view. Note that this is a 30 year old question, and (in my view) modern question stems tend to be a bit more direct.