The cap rule is BYC - Warriors are limited to receiving 150% of Kuminga's salary from last year while any team that takes him has to apply his new contract to their cap. Monk would work if Kuminga's salary is in that range. The issue is Kuminga wanting 30 million and then figuring out how to make that work.
This is kind of silly but the rationale behind it is to keep teams from using sign and trades to skirt the salary cap. It's pretty fair when you consider what happened with the KD situation where they took D-Lo back just to stay as an above cap team.