Maybe yes, my own sense is that in this context C?na points less to a known region and more simply functions as a place-holder for the category of the foreign; in part I think this because the
practices in the C?n?c?ratantra, which are primarily sexual in nature, appear to be rooted in practices popular in Bengal and Western Assam, unlike the practices associated with Eastern Assam, Sadiya for instance, and Burma, known for human sacrifice.
Of course this doesn’t discount your point that C?na might have simply referred to a vague geographic region that these writers supposed to be located where contemporary Burma, Nagaland etc are. I suspect that the Bengali
and Western Assamese writers use the term because it already signified a functional geographic “other”, in this case somewhat denigrated, (maybe similar to the medieval and early modern European uses of the “orient”?), and maybe also not so different from
one of V.V. Dvivedi’s 20th century introductions where he compares the c?n?c?ra practice to Muslim practices.