I disagree. My credentials for this are 12 years nursing experience in a specialist coronary care unit, so I do know quite a lot about cardiac arrhythmia and their causes.
Hypoxia causes ventricular fibrillation. This is the most dangerous cardiac arrhythmia and, unlike most, it always represents a cardiac arrest. Kneeling on someone's neck can induce hypoxia. So even if Floyd died of a cardiac arrhythmia, I do not see how that exonerates the man kneeing on his neck. It seems to me that such a defence could be used of any case by strangulation or asphyxiation: they cause hypoxia which, in turn, cause arrest arrhythmia.
I have not followed the details of this case closely and there may be other issues of which I am not aware. However, I don't see citing cardiac arrhythmia as much of a defence from a physiological standpoint.