Why would Steven explain himself? The point is that he gave himself up so they would leave Earth alone. He didn't expect to go to the tribune because he thought they would punish him immediately.
Having them realize he's just her human son would fuck everything up, they'd likely just shatter him and go to Earth to find the REAL rose quartz.
Yeah the plan has holes in it, but it was all he could come up with when aqua and topaz were going to raditz all those humans.