Considering they probably tested everyone so there are no unrecorded cases, a 1% death rate is pretty much in line with studies finding the CFR to be in the 0.5-1.5% neighborhood [1,2]. I would guess they are younger on average, but the prison population probably has some risk factors in terms of nutrition, insufficient Vitamin D, lack of exercise, etc.

[1] https://www.bmj.com/content/369/bmj.m1327 [2] https://www.medrxiv.org/content/10.1101/2020.04.14.20062463v...