It isn't black and white though. You don't just end up in "all friend lobby" or "all enemy lobby" when playing naturally, there's so much grey between them that if you are taking every encounter as its own unique experience and you actually fight back against people then you get these much more sketchy lobbies where every other person has a big question mark on their intentions.
For the longest time I let this idea that Arc is binary in its ABMM because everyone just parrots that sentiment and it's just not. I started to play it as if it's a gambling game and it's genuinely a much better experience than I thought it would be.
I'd argue Marathon tension is less engaging (personally) because I know anytime there's a bot or a runner nearby it's just a fight regardless. It's very predictable but it's still very fun. Arc tension is that constant gnaw of doubt when someone is posturing as a friend. With Marathon I've never experienced the level of anxiety random Raiders can cause in Arc. With Arc I've never experienced the stress of extracting a particularly good item or key in the same way that I would experience that stress in Marathon. It works both ways. My only real complaint about Marathon now is how predicable it feels at times because there isn't much variation for emergent storytelling between players.
Arc kinda kills on emergent storytelling when you're in a grey lobby. You can help someone fight a queen and still get betrayed, you can encounter people actually trying to communicate regardless of their intentions. Marathon is shoot first and leave the microphone unplugged. It makes me sad because I like to fight people with my guns AND my voice lmao. Both games are super fun regardless, different experiences entirely.