As I said dude, don't post stories written by your so called Assamese historians with no historical value, they are never recognized or cited by the academia.
Now take a look at the map of early 15th century East/South Asia below, published by Harvard University Press, and spot your forest land Assam. Ahoms were nothing more than a primitive tribal kingdom, vying for control over a small peace of hilly area in Upper Assam against their rivals Chutiyas. While, almost the entire Assam, including Kamarup remains under the Bengal Sultanate.
Source: Historical and Commercial Atlas of China, Harvard University Press 1935
Assam has always been a primitive hinterland of Bengal, be it the Palas, Bengal Sultanate or the Mughals. The Mughals were mostly concerned with the Portuguese in the south to establish Mughal naval dominance over Bay of Bengal which is why they remained content with only their proxies in Assam.
Note that unlike Assam, Bangladesh and West Bengal are not forest or hilly areas. Hence, history of Bengal has been written based on archeological evidence and not on folklore or mythologies like what you see among Assamese people.