you have no idea what killed this gentlemen, as you're not a forensic pathologist. The autopsy is forthcoming.
the impact of the asphyxiation? likely significant. Cardiovascularly compromised given his size, an obese gentleman. Perhaps even suffers from COPD? He GAVE the cops a warning; 'I cant breathe'. He said it more than once. Dumb Dumb cop ignored it.
Coroners have said the neck/throat were not damaged. If anything helped the guy die it likely was the knee in the back once he was on the ground, but that's pretty standard procedure.