"Because digital currencies technically accompany a holder anywhere that he or she goes, including across a border, that traveler would need to declare his or her entire cryptocurrency portfolio every time he or she enters the United States."
No. At most, a key accompanies the holder. The currency itself is essentially located on the network.
Scenario: A person has a CoinBase account with 100 Bitcoins and wishes to leave the country with $250,000. This person has chosen to download his bitcoin to a wallet that has no historical transaction history with CoinBase save for a fresh download the DAY AFTER he arrive in a new country...
How is the US Customs going to know where the physical location of that money is or went?
No. At most, a key accompanies the holder. The currency itself is essentially located on the network.