If I am reading this function correctly, both EMP and disarm weapons cause the disarm percentage/timer to go up, although only explicitly disarm weapons count towards the Peacemaker achievement.

Which kind of makes sense in that EMP weapons on their own deactivate the weapons of the target (among other things). If they didn't stack then you would be motivated to try to focus EMP and disarm weapons on different targets which sounds fiddly.

The interaction of a high-damage, low-duration disarm weapon like Racketeer with a long-duration EMP weapon like Widow is sort of weird I guess. But perhaps not any weirder than the interaction of (say) Venom and Widow.