Remember, if the "good guys" got a backdoor access, the bad guys can use that backdoor too. In fact, the bad guys will probably use the backdoor much more frequently, which is why attempts to place backdoor on end-to-end encryption by various governments are very dangerous.

A couple of things to keep in mind, some of which are in the article, some aren't:

TETEA is mostly used by first responders. The primary benefit of first responders using encrypted comm is to prevent unauthorized users interfering with real time communication in a crisis.

While the US military uses TETRA in some overseas locations, it is mainly used to coordinate with local emergency services, and has never been viewed as a secure form of communication.

Lastly, not to be too US centric but TETRA is almost never used in America, where Project 25(P25) is mandated for most originations. P25 was developed after 9/11 as a radio interconnect standard for emergency services and first responders that allows radios to communicate regardless of manufacturer.