Your right that GM should not, or at least, extemely rarely, dictate that the characters are in a specific type of conflict- the characters get to describe how they react to any given to challenge. I was not trying to suggest otherwise with my examples. However, the GM does get to determine if any given action is a possibility given circumstances, and if it is possible, if taking it results in a independent test or a Vs test or a full out conflict, and the all of those decisions may be influenced by if the acting character has an instinxt.
What acting on an instinct allows for, both narratively and mechanically, is for the character to react faster than expected, before any character without a similar instinct, which may result in the GM assigning a different type of test to a certain description of an action than if the character attempted the action without the instinct. An instinct may also allow a character to take an action in a situation where another character without an instinct wouldn’t be able to, as they just aren’t able to act quickly enough. It can even allow characters to act before a planned group action to take an independent action first.
So, for example, two or more players may give similar descriptions on how their characters react to a twist of a group of enemies charging from ambush, and the GM may call for a different type of test or conflict based on if one has an instinct or not. If the group describe their characters running away, but one has an instinct to flee at the first sign of trouble and elects to use it, the GM may rule that the character with the instinct gets to take a stand-alone VS test to sprint away before any one else can react, while the rest of the party will have to engage in a Flee conflict. (It could be that if the character with the instinct fails their role the twist is that they have to join the group for the flee conflict after all).
Or two players may describe their characters throwing a smoke bomb and shouting threats at the charging goblins, where one character has an instinct to intimidate enemies at first sight and the other doesn’t. The GM could rule that the character without the instinct just isn’t able to act fast enough to check the Goblins charge, and that by the time they throw the smoke bomb and start making threats the Goblins are swarming over the party, where as the character with an instinct can throw the smoke bomb as soon as the goblins break cover, and start shouting threats before they take their second step.
Or it could be that the group decides to meet the ambush with a counter charge, with the intent to start a kill conflict, while one of the characters has the instinct to “always attack first”. The GM could allow for the character with an instinct to make a Fighter VS test before the conflict begins to throw one of their handaxes as the two groups close on each other. (This instinct and subsequent GM decision occurred in multiple of the 2e play test groups).
This last example is the type of action I was suggesting in my previous comment- a character with an intimidate instinct could preempt their groups conflicts with an independent action in a similar way to a character with an attack first action is able to throw their axe right before the conflict begins. By acting on instinct, they could try set the group up for success in its conflict by intimidating their opponent. The GM can decide if they succeed at intimidating the group of Goblins if that means they run away, or if they all gain the afraid condition.