The study and the summary do not "assume" that the cause was inbreeding. They both assert that inbreeding could be a cause.

The article says "The team suspects these deformities and tumors indicate a lengthy history of inbreeding".

The paper abstract says "possibly because of an accumulation of expressed deleterious alleles that caused disturbances in developmental processes".

It's perfectly ok for scientists to write down and publish their speculation, as long as it's suffiently clear. I think it's quite clear in this case.