He's jealous. Me and my circle talk Marvel and watch Marvel, it appeals to a broader range of people and has been applauded for trying to make movies more diverse.
It might not be artsy fartsy but those movies get a lot more love and a Marvel movies crew shouldn't be ashamed to receive that.
Its not even really about being "artsy fartsy", that is what I personally prefer but MCU and blockbusters aren't that type of content but they could do so much more because they are extraordinarily safe, which would make them better films. I'm not a superhero fan personally either but I like a good film and I want more than face value, especially on a big budget production.
Give this video essay a watch if you can:
It only focuses on the music but it does a great job of showing how it could be improved, this can also be applied to the rest of the elements in the films such as cinematography, editing, characters (major and minor), movement, relationships, dialogue and themes whilst still giving fans exactly what they want.
I'm not trying to shit on people's enjoyment of the movies either just trying to highlight why many are vocal against MCU when they hear claims such as "the best movie ever made" as they look at film in a very different way. Although I can't speak for the fan wars I couldn't give a toss about that.
I should state that they have also got better in the more recent movies but there is so much they could do especially when they have a wealth of content and resources at their disposal.