UW 3209 says that EMG is not a good choice. It's actually a WRONG answer on this QID as a confirmatory test. While this doesn't negate this card, it does raise into whether it's helpful to link EMG and polymyositis at all.
UW explanation says that EMG is really only useful for differentiating polymyositis from other causes like ALS. It doesn't even talk about it being helpful in making the diagnosis. In light of this, I think removing EMG from this card and sticking with confirmatory tests may be a good option. AMBOSS says that EMG "Can help distinguish between myopathic and neuropathic processes". But the confirmatory test is muscle biopsy on both resources.
I'm deep in IM/Step studying so can't do a deeper dive, sorry!