Lahore has much more population then cities in seraiki belt,so i think the figure must be quite close to thier exact population.
As per official statistics urdu is spoken as first langague by no more than 8% of pakistans population.If the urban punjabis were included in urdu list then i guess the figure of urduspeakers should have been much higher then 8%.
Another thing is that most of the mohajirs who came to punjab mostly got assimilated with the local people just the kashmiris who came to punjab a few generations ago have integrated with punjabis.Shahbaz sharif is of kashmiri ancestry.