I understand what he's saying - and I can't outright say that he's "wrong". Outside of some choice few - MCU films' storytelling is pretty bad. Their characters are the draw. They typically just punch their way out of their problems. The stories don't lend many life lessons. They're escapism flicks, though, and serve their purpose (mostly) for that.
As much as I think Endgame is a fantastic film, it doesn't mean anything. It says, "if time travel exist(ed), then you use that to fix your problems instead of dealing with the weight of your failure". That's not to say it's completely devoid of some lessons, like sacrifice, but it's really just a method of entertainment.
Back in Martin's day, most films had meaningful messages. You can see by all the drek released nowadays that those times are long gone. He's thinking too hard about it, though. Back in his day, the technology didn't exist to make these successful visual spectacles. Escapism can be just as useful a tool as a movie with a moral message.