“despite knowing the patient failed the breathing test”
The poskim who disagree with Rabbi Tendler seem to mostly hold that cessation of breathing constitutes halachic death.
‘misdiagnosed “cardiac death” was a very real concern in his day’
Other poskim hold that cardiac death is the halachic definition of death. That is difficult, as blood circulation was not discovered until about years ago. Chazal would not have created this definition.
“his heart still pumping — albeit assisted mechanically”
A heart transplant can only happen if the donor’s heart is beating and the donor’s lungs are breathing (possibly with mechanical assistance). If your halachic definition of death is anything other than brain death, then all heart transplants should be mamesh murder al pi halachah.
If you go to the heart transplant page at the web site of Montefiore Medical Center, you can see a video of an actual heart transplant.