Not sure I buy that one.
1. A hard R limits the audience that can come to see the film on their own, and a PG-13 rating caters to the underage fan. IMO it'd be the height of arrogance for Sony to make a hard-R film as if they were going to put out their own Deadpool or a Logan. I don't think they'd have made good use of it outside of catering to super edgy people that think maturity means sex and blood and not actual other content or think the material needs to "grow up with them" when it's not about them.
2. The arbitrary standards of the MPAA concerning what gets a rating just makes it that Venom doesn't have any "gratuitous" violence and brighty-colored blood in it, nor lingering shots of dismembered corpses, but the idea of violence happening still sticks around. Like: The Dark Knight was PG-13, and the audience still understood perfectly that the opening scene involved clown-faced dudes getting blasted with guns, or that the Joker shoved a pencil into a dude's eye, and some trucks and things (and Rachel) got blown up. Harvey burnt half to death on screen and they showed his skinless burnt face later in the movie. A PG-13 isn't exactly sunshine and rainbows.
3. The comics that everyone loves Venom in still adhered to the Comics Code for as long as that thing lasted (and he debuted while it was in effect IIRC) meaning the source material was still good to put out to kids and the artists had to find creative ways around showing what they needed to and also getting the issues on shelves. The director of this just had to do the same thing with their budget: do a bunch of takes of symbiotes doing symbiote stuff and find the right one that gets the point across while still being watchable to the undeniable contingent of teens that pick up comics and whose parents are shelling out the cash for. The cartoons and the PlayStation games with Venom in them had to do this too.
They could go fo hard-R worthy content if they wanted to ever make a stand-alone Carnage movie because murder is what *that* guy is all about.
Or I dunno, just show a woman pleasuring herself in the next Venom or one of the minor characters is gay. The MPAA does have its ridiculous double standards.