Where did you hear that, hehe, I never said anything like that, source please. Now lets not bring my personal stuff in it.
I will tell about a friend of mine. His name is Khan -----. This guy actually had a legend in his family that some foreign gentleman came down centuries back, married into local family and founded his family. He is medium dark like average Bangladeshi, but his facial feature and body structure look distinctly mixed, Bengali local with proto-Turkic, almost similar to Mongols and it is nothing like Chakma or North East states tribals. So this is just one example, these kind of examples are all around us.
The thing that you guys forget is that Eastern Bengal was a sparsely populated jungle area. It was settled by clear cutting teams headed by migrant Muslims and local converts working under them. Most educated land owner families descend from this mixed ancestry, while the land-less tenant farmers are mostly local converts, but then over centuries fortunes changed and people got even more mixed.