From the article: “The medical examiner’s report could bolster the defense team’s contention that the officer’s actions did not play a significant role in Floyd’s death. But it might be challenging to prove that he would have died at that moment even if he had never encountered the four men.”
The defense doesn’t have to prove anything. It is up to the prosecution to prove the officer’s guilt beyond a reasonable doubt. There is always the possibility that he died from a drug overdose, not by anything the officers did, so I do not think that standard could be met.