The exact wording in The Guardian is as follows (my emphasis):
Agent Findley and several others repeatedly asked Tunick to open his phone during the interrogation, telling him they would seize it if he did not.
In my opinion, this means that at the moment he provided the PIN, the phone had not yet been seized and was therefore not yet under the control of the government. Again, in my opinion, this means he did nothing illegal because it's the same as if he'd wiped his phone as soon as the plane took off. In either case, the phone was not under the control of the government and was his to do with as he wanted.
The agents should just have seized the phone to begin with and let forensic investigators figure out how to unlock it later on.