The numbers are correct, I just wasn’t clear in presenting them. There were 75 starters who had a 5 ERA or higher. They combined for 4373 IP or 145 IP per team, not per player. Per player that averages out to 59 IP.
No team had 1 singular fifth starter. They all used a combination of pitchers for that 5th starter. 75 of them had an ERA over 5, they all combined for 4373 IP and a 5.86 ERA. So the average 5th starter slot had 2.5 pitchers filling it, with a combined ERA of 5.86. Hope that makes it clearer.