I’m not sure what “set this up as a combat encounter” means. Between sessions, I prepped simple stat blocks for the NPCs — the two fighters and their seven hirelings. When the session began with the party opening the door to the room as had been previously decided by the players, I made a reaction roll for each of the two fighters, the result being they both had a starting attitude of Hostile, but not violently so. I telegraphed their hostility by describing the fighters making a claim to the dungeon’s treasure as rivals to the party and implying their willingness to back up the claim with force if necessary.
So the purpose of the encounter in this case is to provide opposition to the party, which I see as one of my primary jobs as DM and which I touched on somewhat in my OP, rather than merely being a venue for combat that the player is "getting around" by interacting with the NPCs socially. The party overcame this opposition by employing a deception, but if the check had failed, I would have followed through with having the NPCs initiate combat as a consequence for failure.
ETA: Maybe that’s what you mean by “combat encounter”, but I’d contrast this with the encounter with large spiders earlier in the adventure in which each spider had a very high probability of attacking the party on sight.