The defense does have a murky cause of death to work with in the sense that there were multiple factors: heart disease, drug abuse, excessive adrenal output, resisting arrest, delirium/panic attack, and more.
The prosecutor addressed that by saying Floyd lived with all of the pre-existing factors until he was arrested on May 25, but I think there is plenty there to muck up the COD and create reasonable doubt on that issue. The defense saying that there was no bruising or petechiae in the neck tissue goes against mechanical asphyxiation as the COD.
But I just keep coming back to the 4+ minutes the knee was on the neck after the man was quiet and motionless. That's going to be a tough thing for the jury to not apply reason and common sense to.