I think the study makes a giant presumptive leap to assume inbreeding was the cause of the higher-than-modern-human incidences of spinal tumors. It's irresponsible to assert this to be true instead of one of several possible explanations.
Another predator that went through a genetic choke point. Cheetahs did, too.[1]
Cheetahs survived, but they are so similar genetically that they get the same illnesses, and skin grafts between unrelated cheetahs are not rejected.
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.
[1] https://academic.oup.com/jhered/article/108/6/671/3836924