This anagram has been developed by losers really. There was no anagram when Pakistan was coined or created.
Bengal was always going to be the part of Pakistan and everybody knew it (except the losers of course).
But Bengal couldn't remain Pakistan because the theory of "Muslims will live peacefully with Muslims only - and not with the Hindus" was so flawed that it couldn't last for more than 26 years.
Bengalis mutined, we just provided the support as the civil war in what is now Bangladesh was going out of control. Too many refugees in india. And you provided us a wonderful opportunity when you attacked Agra. We were waiting for it, and we ended it swiftly. That's what Modi is talking about.
Please read this article published in your newspaper:
In a signed preface to India 's Problem of Her Future Constitution, as early as October 7, 1940, the Quaid-i-Azam said, "That the areas in which the Muslims are numerically in a majority, as in the North-Western and Eastern zones of India,
should be grouped to constitute an 'independent State', in which the constituent units shall be autonomous and sovereign."
https://nation.com.pk/22-Oct-2008/state-or-states
If you read the article you will find out that that 'the concept of Pakistan' included Assam too!
But anyway, I just wanted you to know that Bangal was indeed the part of "Pakistan Idea" as early as October 1940.
I'm sorry that the two nation theory couldn't last for more than 2 dacades though. The war is always unfortunate. So don't take Modi seriously because he will ofcourse praise India's role during the 1971 and would want to score points in Bangladesh. I'm sure you would've done the same if the result was reversed.