You seem to contradict yourself here. I don't really believe his excuse but if it did effect him then it would have effected him from the start since he was carrying all that weight in.
What I mean is it surely had an affect on his performance overall, but the difference wouldn't be so great as to cause him to get dominated from the first round.
Deontay Wilder is a world class athlete, carrying around 20kg isnt a huge amount of weight especially when distributed over your entire body, even if he was wearing it for 15 mins.
As such, I can see how he would be slightly tired in the first round, but not to the degree that he should get completely smashed by FUry from the first 10 seconds of round 1.