I would assume that the author's acknowledgement that the Europeans did not know (nor could they have known) that they were carrieng diseases that would kill the indians does mitigate their "responsibility".
At some point the indians were going to be exposed to these pathogens, and had it happened any time before the 20th century, the results would have been the same.
There’s plague still resident in rodents on the US west coast and southwestern US deserts.
Plague came to Europe from the east. I’m guessing that it also came to the western America AO at about the same time via ocean travelers.