On the same page 13 I think you missed THIS bit..
The majority of those who had migrated to West Punjab (Pakistan) had moved from East Punjab (India). As such there was no problem at the level of culture or language and hence, integration.
The point is both you and Aliyish are right, Aliyish is refering to URDU SPEAKING immigrants from UP, CP and other areas of India (Generally THEY are refered as MUHAJIRS), and MAJORITY of them migrated to Sindh. If we include Punjabi Speaking migrants, then total of course comes around 70% of mass migration.
Yep i do hear this argument all the time. But the question is why the integration did not happen? Was it because the Sindhis were too proud or racist and did not accept migrants into their society or was it because migrants considered themselves more educated and better than locals and did ntio want to get mixed with locals.
By the way, there were still lot of differences among culture of east and west punjab. My maternal family migrated from east Punjab. For a long time the migrants from east Punjab were called "mahjar" in west Punjab.