I can't ever imagine playing this game solo being fun because it's designed for co-op. As a co-op experience it's a blast and you can have a laugh. It's never going to be gaming's Citizen Kane but then it doesn't need to. As a shooter the gameplay is solid and there's a good variety in enemies and situations.
In terms of horror, nothing will make you yell, but likewise I do think the Supermarket/DIY store and the bridge train bus thing with the lickers in were good at creating an unsettling "what's going to happen" atmosphere, although there wasn't really a payoff.
I note your review focuses a lot on the story from a fan of the series in general. When myself and DK played it we had no conception of the story and didn't really pay attention at all. There's a mute Solid Snake and his ghost brother and they're fighting robots and weirdos and taking over giant mechs just 'cause. Plus there's some woman called Alma or something? Who knows? Who cares? I'm a ghost!
The multiplayer sections were also pretty cool. This, for me, is really where the horror is. Generic Horde mode with base building elements is decent enough, but the added danger of "OH trout IT'S ALMA DON'T LOOK AT ALMA!" as a little girl wanders about is a good addition and the horror - if one can call it that - is the feeling of dread that at any moment all your hard work is going to be undone because you glimpsed at this random enemy that can pop up anywhere, at any time, and you can't fight them or stop them.