The ratio is a little unusual but the math is right, the divisor is per days worked.

15 days off / 20 days worked (5 weeks of 4 day workweeks)

8 days off / 20 days worked (4 weeks of 5 day workweeks)

Another way of saying it would be: in a 4 day work week you have 4/3 (1.33) ratio of work days to days off whereas in a traditional workweek is 5/2 (2.5). So although it is just one day it dramatically shifts the balance between work and time off.

Oh, I see! I thought they were saying they had 15 days off in a 20 day period, not in a period when they worked 20 days