Though the phrase 'true fans' does admittedly sound like a load of gatekeeping BS (something geek culture has always failed to deal with), comic book movies will never not be for everyone. To argue otherwise would just be plain silly.
That aside, hidden within these big budget adaptations are references, details, and homages only the hardcore of fans will be able to find. There are the obvious easter eggs and references that have found a place in pretty much every MCU film going, but other features have gone a step further to pay homage to classic comic book story arcs, replicating iconic sequences panel for panel or even entire beats from certain storylines.
Of course, if you don't read the comics, these kind of details are likely to fly over your head. For those who do, however, these moments come as a genuine surprise and provide further impetus to speculate endlessly over prequels, sequels, spin-offs and the rest. Further still, comic book movies only benefit by sampling the best the medium has to offer - you only have to look towards Christopher Nolan's Dark Knight Trilogy for confirmation of that.
There's a lot to be said about how filmmakers develop the 'perfect' adaptation, but in looking towards the best and brightest elements from the comics medium, not only do they satisfy that hardcore element, but they're left with a better film too.