It's a valid point but the boarder issue long predates the Chinese plans to move lots of water .
It seems grounded in a stand-off between boundaries set out (or not) by "unequal treaties " and imperialism on the Chinese side and the Indian view of sacred country none of which can be disposed of.
The Brahmaputra runs for hundreds of miles through what is clearly Chinese Tibet so they don't need to fight a border war to be able to divert it (if they wanted to). The main problems with the border in the western sector are someways north of the Indus in the Aksai Chin. This was more to do with the British trying to push the Imperial Russian Empire northwards than anything else.