Not at all. The "I wasn't planning on it" answer is, essentially, a commitment that an attack won't take place unless something significant changes. That is more ambiguous than a straight "no", but if the asker isn't planning to change anything significantly, then it's nearly as good as a "no", and a lot better than no answer. If the asker IS planning to do something significant, then it is probably inadequate from their perspective and the dialogue will presumably continue until either an agreement is reached or negotiations are abandoned. In my experience, the vast majority of the time the asker is not planning to do anything significant and "I wasn't planning on it" satisfies them.
If the ulamog player wasn't planning to attack them in the first place, then there's no dealmaking necessary. Ulamog player just answers "wasn't planning on it" and things presumably proceed amicably. If that's inadequate for some reason (perhaps the asking player is planning to do something semi-significant and wants assurances that it won't change ulamog's mind) then they can get into those weeds if they want.None of the measured communication you're offering solves the position of "if you're going to attack me with Ulamog, I'm going to kill you first." The only way to placate that is going to be dealmaking. You can go into infinite permutations of what situations you may or may not attack them with Ulamog, but they're asking the question because they want to get you to not attack, and are going to choose an option that reaches that result. That decision has already been made, the rest is just bargaining with more words.
Assuming that the asker absolutely cannot handle an attack from ulamog and would kill the ulamog player if that was a threat, then yes, from their perspective the negotiations will always end in not getting attacked by ulamog - either via an agreement or via killing the opponent. But how that plays out depends a lot on the board state. If attacking the ulamog player for lethal will guarantee them getting killed by the third player, then they have a very weak bargaining position. If they could kill the ulamog player and still have a strong chance against the third player, then they have a strong bargaining position. So how many caveats/demands the ulamog player can reasonably extract from their agreement not to attack will depend upon those factors. If the asker's position is too strong and will only accept a "no" with no caveats or demands, then ulamog is probably dead either way and so they might as well decline to answer - but that's only true on the far end of the spectrum, not across the board.
I don't like this dichotomy. It's a threat in the game. That carries zero percent of the moral weight of making a threat irl.
As far as how the other person perceives it, it's probably going to come down to tone and specific word choice and a bunch of squishy human factors that are difficult to examine with any real rigor.
I do agree that ultimately most questions are implied threats within the context of the game, but I don't think that really means anything in practical terms. A threat is just another kind of move, it's not inherently malicious like it would be irl.
I think drawing parallels to real-life interactions are going to lead you to wrong conclusions.The question itself engendered hostility. If insight into their perspective was the goal, the wrong question was asked. If somebody asks "do you wanna fight me?", the hostility isn't coming from an answer. You wouldn't look at someone answering that question and say "well if you didn't want to fight, you should have just said no, you didn't have to make it hostile" when they try to walk away from the question.
This is a game where everyone is trying to kill each other. "Hostility" within that framework is not avoidable.