Blocking individual critics from press screenings occasionally happens. Though it’s very controversial and it tends not to work out too well for the company doing it and most don’t.
The thing being exaggerated here is the same access journalism problem that hits most press coverage, and is measurably worse in other fields.
For the entertainment business this takes the form of positive coverage of shows and movies, to maintain access to things like celebrity interviews, photos. And for the purposes of popular movies, tempering coverage for early promo material. Those set pictures, cast interviews, set visits, early footage and early detail releases (like say the title of the new Star Wars, or casting info).
But most critics don’t do that sort of coverage, or even work for publications that do. So they don’t need to engage with it. It’s mostly a feature of your glossy celebrity gossip tabloids, and publications like Entertainment Weekly. That make their nut by having the stars be just like us and occasionally getting the cast of the latest tent pole on the cover. Though it does impact fan and pop culture publications (like Scifi Wire referenced up thread) that specialize in covering genre material. The former don’t generally offer reviews at all, instead offering positive blurbs where everything is awesome. And the later, while they may fear it, haven’t generally found themselves cut out due to negative reviews. You need look no further than the fact that these places are still welcome at press screenings and still get info from WB and Fox, despite regular extremely negative reviews and coverage of shit like 4tastic and Batman vs Superman: Skull Humpers.
So the claim that it has a material impact on agregate critic scores is frankly, conspiracy.
And yet the claim is usually that RT is disadvantaging WB and favoring Marvel.
Which doesn’t have much bearing the movie. Guardians had only recently been relaunched when the movie was announced and it was cancelled due to low sales shortly after. And the original run was never particularly popular, didn’t last long, and had been pretty much forgotten at the time.
And these days the comics are more merch for the films than anything. Successful movies drive sales of comics not the other way around. The “built in audience” aspect of this has been inverted. And the whole fans being entitled to XYZ thing has never really held water. Generally these companies do not owe you anything, and you don’t get (nor should you have) a creative say in what they produce.