It is generally agreed that the majority of residential-school deaths were caused by tuberculosis or other disease (Many of the children arrived at the schools already infected with tuberculosis. And rates of tuberculosis, virtually eradicated in the general population, remain high amongst certain Indigenous populations, rising to 300 times more for the Inuit).

Yeah, that should explain it.

Many of these deaths may have been avoidable under better living conditions or more scrupulous care in segregating the sick from the well (a lesson the shamefully high COVID death rates in our nursing homes proves has still not been learned). It may be disheartening to know these poor children's bones lie in perpetual anonymity, but it should not be shocking to us that the graves were unmarked. As Champion notes, unmarked graves "were the norm for the poor for centuries: in the Irish potato famine and during the Spanish influenza."

Spanish Flu ravaged Poland in general (the country utterly pillaged and scorched-earth destroyed in WWI by the both sides going back and forth, and semi-starving, and still at wars) but especially killed off the Bolshevik POWs in their camps. The guy mentions Katyn, so I'll also do to tell how about several years ago the (often supposedly anti-communist) Russian nationalists started a public theater of crocodile tears about this to equate these deaths with their 1940 shootings.