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.

It’s not an issue of him ever thinking of choosing to wipe it during his travel, he clearly had no initial intent of doing so, which is why he didn’t do it before departing . What’s caused all the legal issues is that he decided to give his duress pin when his actual pin was demanded by the US border guards. The border guards entering that pin caused the device to reboot and factory reset. That’s why he’s in trouble. They’re claiming he purposefully caused evidence to be destroyed when an investigation was already underway.

It's not that crazy. Do you wipe your phone every time you travel? It's the most secure but least convenient option.