i don't think that's what cleavage means? doesn't it just mean yknow, visible boobs, which they are? isn't seeing the bottom which is obscured by a shirt called "underboob"?
He's complaining that she's wearing an undershirt - the orange "cleavage blocker" under her white partially unbuttoned shirt - and it is obscuring his view of the buttcrack cleavage he expects to be on display for him.