Coaches tend to make less than most players.
Any money he made was made as a player before he transitioned to coaching.
Yes, they make less than top players, but they don’t make nothing. And not all players make so much. Pierce was an undrafted player, who eventually played 9 years, and was an alternate in pone Pro Bowl. I doubt he made as much as you think, and I doubt it was $28 million. They didn’t have info on Sportrac.