It’s not true they can’t determine his intent without talking to him. The Manual for Courts Martial says that circumstantial evidence can be used to determine intent to desert. Such things as prior to departure taking care of possessions or finances are among those actions listed that can be used to determine intent.
Just so everyone knows - for them to start any UCMJ procedures (ie courts-martial) he has to be returned to active duty first. Not saying this is what they are doing, just a fact.