Complete nonsense. The "stressed output transistors" are part of the audio amp, and have nothing whatsoever to do with the transmitter section of the radio, other than being connected to it. There is no way on the planet that a failed audio output transistor could interfere with someone else's transmission... quite apart from the fact that pretty much every TSO'd radio has output transistors with full thermal and overload protection. And in any case, if those transistors were to fail despite the protection, they would almost certainly fail open-circuit (ie no output at all).
Hmm! We're an avionics technician are we ?
Remoak, the failed output transistors would render the comms unservicable due to NO audio output.
If ya can't hear ATC then ya can't do much with them. Interference isn't the question.
So while it's true that the POH probably doesn't specify the exact type of headset, it does imply that the comms will be used as per the manufacturers specifications.
You will find a listing of impedance ranges there if you look.
Are you suggesting that you are free to plug in a shorting jack into a headset output ?