A far better approach is for the US citizen to simply say "I chose not to provide my PIN".

The officer will say something like "That's your choice, but I will need to seize the device to conduct an analysis. It will be returned once the analysis is complete".

Then you shrug, and they will let you enter the US. The cops will try to get into the phone, fail, and return it to you.

Just bring a phone you don't mind losing for a few months.