This. A charge isn't a conviction, though.

Plausable deniability might help you here as long as you can think of a valid reason why you had that code on the note. It could be as simple as "this is the code I needed to remember for this one game" or something too. As long as you don't tell them that the note contains the code to your phone, you can pretend the note contains a code of some other value instead later on and let them shoot themselves in the foot.

As a general rule, most lawyers would tell you if you are under arrest to say nothing more than what you are required to by law. Since this is at a border, it gets weird, but that's what makes this case unusual as well. Under normal circumstances, they might seize the phone as evidence, but saying nothing might result in them wiping the device themselves before they ever get a warrant for the passcode.