How is that a test of GP's hypothesis? Their claim was about commercial spam or scam calls, not potentially felonious threats.
It seems to me the claim is that identifying VOIP subscribers is not feasible.
Feasible (as in practical in general to identify spammers/scammers) and possible (as in "could a concerted legal effort identify a single caller who made a threat against an official?") aren't the same thing.
1. Spin up new voip company
2. Issue death threat to US President (or similarly powerful man)
3. Spin down voip company
4. Wait to see if/when someone finds out who you are