Mechanic have much More DPM Than sairs -.-

Possibly he means mastery giving +20 WA at max.

Only reason mechs have that much passive WA is they use guns but not bullets, so they have to make up the attack somewhere.

I think the justification is that Mechs dont get any sort of built in crit like corsairs or +min crit damage like archers and hermits built into their mastery. So instead they get a free 20atk boost (past the 20atk boost to make up for no bullets). The +30atk from extreme mech simply mirrors a bowmans' 4th job mastery.