From a pro-privacy standpoint changing your passwords to something you can't remember is absolutely the right thing to do to prevent yourself from being compelled to give up any passwords...
But in light of him being a government official tampering with his government issued phone to "accidentally" prevent it being used to investigate official wrongdoing, that act becomes highly suspicious, and (in the FBI's shoes) would only give more incentive to hack into his phone.
As someone who has worked with academics, the more specialised the person, the less common sense they seem to hold onto.
As such, if this was outside their PhD specialisation, then it'd absolutely make sense that this wouldn't occur to them.