I mean, I’d rather just live with the results, whatever they may be. But if the DM feels guilty about accidentally making the fight harder than they meant to, being honest about it is better than deceiving the players, IMO.
D&D is not such a convincing illusion to me that this would make a meaningful difference. I know the DM (or the author of the module) planned the encounter (or seeded the random encounter table). I know they are an imperfect human being who sometimes makes mistakes. I know they are trying to make an enjoyable gameplay experience for all of us. If they messed up in doing so and want to fix it, I would rather they own up to the mistake and have a human conversation about what to do about it than try to go behind my back and rob me of an organic gameplay experience without my knowledge or consent.