I think the CEO of Toll Brothers is probably right – they can’t go much lower than their costs.
However, cost does not equal value. Values can and do go lower than costs. That risk is why developers can readily make a lot of money when times are good, and can just as readily go broke when times are bad.
BTW, costs are dropping, too, not that it much matters.
As for Diamond Bar, that area is closer to significant employment than points east. Your trend for correction will continue to march in from the periphery. It’s not a question of if, but when.