Smaller nm mask means more hashrate per chip. However, they're more expensive to produce as newer fabs charge higher prices as they're still recouping their own investment. Friedcat guessed right that an antique mask would mean he could get the chips fabbed faster with greater reliability. He also could lock in the fab to produce them in volume for him.

At some point AM will have to go to a smaller mask, but at least now there's a bulging war chest to pay for it, I expect. So booking a newer fab for a longer period of time becomes feasible.

There are some other companies already developing chips with much smaller masks (~35nM maybe?) but none of them have hit production yet AFAIK.