He might have a point in that having not just another giant superhero film but one with a very similar premise of hero vs hero come out and do poorly less than 2 months before CW came out might have cooled some of the GA on rushing out to see this quite as fast and that might have lessened the OW for CW.
I really don't buy this.
Civil War had an A cinemascore, which means it was received by audiences relatively similar to AoU, to Avengers A+. Despite the critical discrepancy, this is playing like two similarly received movies.
While I think all of this gloom and doom about Civil War's international performance is just that, fan boys being fan boys, I think the harsh reality of Civil War's domestic performance is the lightning in the bottle scenario.
Avengers was the first time we saw the team assemble. It was the first time we saw a Hulk unhampered by budget constraints. The whole thing fresh, and had a zeitgeist to it that the second and third time out lacks.
It could just be that $450-500 M is where the general audiences interest level is for these characters. Maybe Infinity War will push the needle back up, but there's a chance that domestically, Avengers 1 was a peak that the MCU won't see again.
Truthfully, there's nothing wrong with making movies that do "disappointing box office" in the $1.2-1.4 B range, and "Only" break into the top 10-15 of all time domestically. I don't see sequels for Avatar, Jurassic World or The Force Awakens doing nearly what those films did either. The next big thing is always going to be something new.