Taking a bad gas tube off and replacing it (which is what I'll bet he actually did) isn't exactly a routine cleaning operation, it's upper echelon maintanence and repair. Built more than a few ARs myself over the years and have been using them extensively for about 35 years now. There was definately something else going on with that rifle.
I was standing 2feet from him when he cleaned it. Watched the whole process. He did not change anything.