“despite knowing the patient failed the breathing test”
The poskim who disagree with Rabbi Tendler 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 been able to create this definition.