The vulnerabilities allegedly exploited in the article (in SS7) have nothing to do with encryption being backdoored.

It’s a bit of a weird conclusion to make…

Maybe they are saying that if the PSTN ran with strong encryption and authentication and that encryption wasn’t backdoored, this thing that is alleged to have happened wouldn’t have happened ?

SS7 may or may not have been intentionally backdoored at the time, but its development also predates the invention of the RSA algorithm upon which all modern security is founded. A key–exchange system like RSA or its successors is really required to do end–to–end encryption, and some sort of CA system is necessary if you are going have any hope of verifying that you are talking to the correct phone on the other end.