If anything, Kimbrel just maintained what the overwhelming majority of athletes would maintain - that he feels fine. Guys do it all the time, in every sport. They honestly usually even believe it.
It’s on the staff of a team - be it coaches or medical guys - to identify that hey, maybe this guy is not, in fact, fine. I don’t think anyone involved today was making selfish decisions. Kimbrel thought he was a little off - as players sometimes are - and that he could get back into the groove, and his manager believed him.