While I agree that the author’s invoking of this particular incident in which TeleMessage created an insecure side channel for message transmission and storage which undermined Signal’s E2EE security doesn’t really provide proof to their claim, they’re not wrong about a master-key encryption system being an absolutely horrific idea.

Did you read the article? The author manages the press department of Tuta Mail, an E2EE service. The original Signalgate wasn't about a cryptographic backdoor, this one definitely is.