I think this alternative answers Exercise 5.7.8 with fewer lines of code. Note though that the answers don't coincide, i.e., I don't havet any tailnum with zero flights before the first 1-hour delay:

flights %>%
  filter(!is.na(arr_delay)) %>%
  arrange(tailnum, arr_delay) %>%
  group_by(tailnum) %>%
  filter(cumall(arr_delay < 60)) %>%  ## all until this 
  condition is false
  summarise(n = n())