if the US actually has laws that allows it to prosecute him in this scenario.

Big if.

Experts said the legal approach is unusual and may be the first time the law has been aimed at an operating system. “It’s concerning – and sends the message that [GrapheneOS] is criminal by default,” said Christophe Boutry, a cybersecurity and surveillance expert. Boutry and Bill Buddington, senior staff technologist at the Electronic Frontier Foundation, both said they had not seen a similar case.