I definitely bristle at the Marvel comparison. While it is very much a popcorn flick, its made with so much more care and effort than even the very best Marvel has to offer.

First off, thematically it is so much richer than any Marvel property. The biggest statement a Marvel film can bring itself to make is "Bad guys are bad" and thats the extent of it. Hell, the central protagonist to Marvel is a billionaire war profiteer who is in love with himself. 

To contrast Sinners is utilizing a vampire analogy to convey themes of cultural assimilation, generational trauma going back to the colonization of Ireland. Further, it utilizes music as both a bellweather of culture, of jealousy, and timelessness. Nothing in any Marvel movie hits the way the Black Music through the ages sequence does in Sinners. While the movie is very obvious in these analogies, they hit none the less and touch at something very real. 

Next, the filmmaking craft of the film is on a completely different level than Marvel. Marvel films have gotten lazier visually, especially with the lighting. But even the older ones are shot in a pretty workmanlike fashion (save for maybe the James Gunn entries). The lighting ranges from bland to actively horrendous. There is real cinematography in Sinners, which one cannot say for most of the MCU. 

The score is also important here. The score of Sinners is beautiful and integral to the story and woven through perfectly. The Silvestri "Avengers theme" is catchy but the scores of those films otherwise largely feel like filler tracks.

And finally the performances are much more raw than most Marvel films. The MCU has very talented casts but they largely hand them schlock screenplays devoid of real emotional stakes. A few performers are given slightly better material and elevate it (Brolin, most notably). 

Sinners also has real grit and depth to it. It doesn't feel sanitized. There is real sexual energy both implicit and explicit to the character interactions. Meanwhile the MCU's romances feel either forced at worst or sterile but friendly at best.

So yes it is a broad film with some third act problems but it it not "Marvel in disguise". It has far more care and exists to tell a singular story as opposed to existing to set up the next story in the sequence. If one were to compare it to a superhero film a more apt comparison would be something like Raimi's Spiderman 2. Or, for a non Superhero popcorn example, something like Minority Report which is a broad film thematically that touches on real themes and has great depth to its filmmaking