Uh, luck. New drums actually do nothing to cure squeak. The initial RMS of the finish may help slightly but that is worn off in one hour of use. There is no specific 'machining' done on drums to make them not squeak.
Focus rear drums squeak because of the thin backing plate and the way the shoes anchor down. Brake dust makes it worse, dry lube points where shoe touches plate make it worse. Fix either of those and problem goes away for a while. Actual problem is that the right angle end of shoe material digs into the drum to lock and twist until drum motion snaps it back loose to repeat again. The high frequency of it makes the squeak. A function of semi-metallic shoes, metal on metal always tries to squeak. If you knock off the right angle corner of shoe to not dig in anymore the shoe cannot dig in to make vibration. Works on any and all shoes I've used on Focus and takes maybe two minutes to do with shoes out of vehicle. I've got two sets of the loudest squeakers you ever heard on two cars and they haven't squeaked once since I fixed them using exact same brake shoes/worn drums, that was long ago............