Is this game really NTR or people just assuming it is? Characters being promiscuous and having no in-game confirmed relationships does not make a game NTR (even if the players themselves feel that way).
No, it's not the literal definition of NTR (but likely fits the loose one many folks seem to use casually in place of anything that sort of fits cuckoldry in any form), it sounds like the player "may" simply get cucked in the game. (you yourself made a statement I'm going to quote momentarily that basically summarizes the concern a number of folks have with this)
What I don't agree with (if it's happening) is the devs not being clear-cut on what the content will be about.
This is basically the concern. They've been pretty vague on this part of it and whether it will be "canon" in the game that the girls are doing the monsters - they've always stated when asked about xenomorph stuff that all monster stuff will be optional as many folks aren't into that...but then looking at posts like this one with good references:
https://f95zone.to/threads/subverse-promising-h-game-on-kickstarter.27806/post-5128675
it gives the impression that there will be some canonized incidents of the girls doing this stuff regardless of the player's desire to have it in the game or not; so there will potentially be incidents in the game or just conversations that indicate the stuff is happening behind the scenes and the captain is effectively "being cucked", which some folks are fine with, and some won't be fine with. As you said, they just need to be
very clear about this stuff - while it's not NTR as defined, it's still an area that a lot of vanilla folks who just want to romance beautiful waifus and / or have a private harem are potentially going to feel a bit misled by, especially since some players get attached to their waifus and don't want to share them, lol.
Like I mentioned in an earlier post above (
https://f95zone.to/threads/subverse-promising-h-game-on-kickstarter.27806/post-5135065), there's just conflicting marketing material out there - they talk even in some of their official dev diary entries about full fledged romances, etc. which, at least to me, seems to go against having the girls canonically running around fucking monsters if the player doesn't want them to.
So guess we'll just have to wait and see. I'm hoping a copy will hit the high seas so we can check it out and make an informed decision before pulling the trigger with our wallets; this will avoid people potentially being very disappointed in the content and then ranting on the Steam messageboards. If I could confirm there is zero canonized monster stuff in the game I'd buy the game as soon as possible, but not gonna do that until I know for sure and if it has this type of content then I'll know to stay away.