How was that subpar? He is in the right position 90% of the time in that video.
There were a few instances where he just got flat out beat by Durant, and some other times where he was late with his rotation on the switch, but overall, he was almost always in the right position and kept his man in front of him.
The simple fact is, LeBron was overwhelmingly outgunned in that series, and the rest of the Cavs were such atrocious defenders themselves, that LeBron was constantly being put in difficult positions or being asked to cover for his teammates.
I don't think this video is evidence of poor defense on his part in any way. It does seem to suggest there is some slippage, and we've seen that in the regular season he takes plays off when he never used to, but that definitely wasn't poor defense in that video.