The thing is, Seraiki itself is hardly a homogenous language and has a lot of variation/dialects in itself. Derawali spoken in DG Khan/Rajanpur differs from 'Riasti' of Bahwalpur and Rahim Yar Khan. Not to mention the nomadic tribes of Cholistan that speak some Marwari dialects. Moreover, these Seraiki nationalists tend to claim way too much of Punjab with maps like these
All non-Majhi dialects like Jhangochi, Thalochi, Chenaweri etc. get claimed as part of Seraikistan. It also has no geographic coherence, stretching from Mianwali to RY Khan, wtf.
Parts of Northern Sindh and Eastern Baluchistan speak Seraiki or dialects similar to it as well, 'Siro' is what South Punjab is called in Sindhi (it means North), and the people inhabiting the region 'Sirai', hence the language, 'Seraiki'.
