I think it's misguided for the paper authors to use the descriptor "non-binary" for this person, as while you could argue that in the literal sense of the word that does appear to be the case, the way we use the word generally is in reference to a culturally-specific phenomenon.
If you read the whole paper, filled as it is with qualifications, talking about uncertainties raised by the excavation site, and cautioning against making assumptions, to have put "The overall context of the grave indicates that it was a respected person whose gender identity may well have been non-binary." in the abstract looks more like politics than academic rigour. It's a shame, especially since they resisted such headline grabbing in the conclusion:
"The Suontaka burial has previously been interpreted as a woman buried with two swords. The grave is exceptionally equipped, with several signs of wealth and care, including prestigious furs and rare feather bedding. However, only one sword, hiltless and silver-inlaid, can be conclusively associated with the burial assemblage. The bronze-hilted sword was possibly placed in the grave some time after the burial. Our aDNA analysis suggests that the individual may have had an aneuploid male karyotype XXY (Klinefelter syndrome). These new research results indicate that even in early medieval Finland, which is often considered a masculine and warlike society, there may have been individuals who did not fit into a binary gender model. These individuals could also have been respected and considered important, and their gendered identities could be elaborately represented and remembered in the content of their burials."