I.e. if they can only manufacture 1000 widgets, but 10,000 people want to buy one at the given price point, the gained market share is still only 1000 individuals. The number of people who want one but can't get one due to lack of supply is not market share.
Temporary under-pricing might be a valid strategy if AMD is planning to ramp up manufacturing capacity, but comments further down in this thread suggest they lack the fabs to do that.