You are right, the authors of the paper only claim that their results demonstrate a); they are, however, assuming b) based on previous studies.

What the Professor is arguing, as I understand it, is that those previous studies do not necessarily warrant the assumption of b), and thus the paper's title "A female Viking warrior confirmed by genomics" is misleading -- there has been a confirmation of "female", but not of "warrior". Something like "Skeleton, that some evidence points to being that of a warrior, was confirmed to be female"; but that's not as pithy or sensationalist.

Those not reading the paper sufficiently carefully (or at all -- only limiting themselves to the title and the abstract) will end up drawing conclusions unjustified by the paper. Which, given the political implications, may be undesirable.

[edit: grammar]