It’s not a myth and not completely true either. Look at the map of undivided India. From the western border of current Pakistan to Bangladesh, from Indus plains to Ganga plains to Bengal Delta, people show a continuous variation ethnically. Western part of this region, today’s Pakistan and Punjab, Himachal and Haryana in India are quite similar, all this region is west of Yamuna which was a significant cultural divide. East of Yamuna UP starts, accent and looks are a bit different. Move along the same direction, physical features in people further change till you reach Bengal. The point is that it’s a continuous, smooth change. India Pakistan border came much later and is only an artificial divide.

Idk what history they teach in Pakistan but North west and north India saw so many immigrations in history. Pre Aryans, Aryans, Turks, Mughals, Jats and many more. You’re not wrong when you say Arab influence is there in genetics but you’re absolutely wrong if you say that is the only source of your population genes. Pakistan and North West India is a region is where most racial mixing took place compared to any other part of the world in the entire history.