He specifically said I defeated and humiliated the players. That's simply not true. [. . .] The most you can argue is that it's possible my players perceived things that way.
I'm sympathetic to your situation, and I think the way you've responded to advice on this thread is solid. But I think you're splitting hairs here in a way that's damaging both to your position in this discussion and perhaps to the solution you're trying to reach with your players.
As a GM, more than in any other social situation I can think of, you are disproportionately responsible for others' perceptions. Your players are highly dependent on you in forming their perceptions, so whether or not you intended them to react badly isn't really relevant. The point is that they did, and that, as GM, the solution to avoiding those reactions in the future is mostly in your hands.
I think it's definitely worth clarifying to your players that you did not intend to humiliate them or box them into a corner, just to preempt any potential bad feelings. That, along with the other things you've already discussed, will help set better expectations going forward.
But I think you also have to acknowledge, if only to yourself, that when players pop out of "character" mode and into "player" mode, they might longer see the situation as their characters against the dragon. They might, if only subconsciously, see it as them against you.
Are they right? Of course not. But I've had gaming groups break up over this phenomenon nonetheless. Avoiding that sometimes requires the GM to accept a higher level of responsibility in managing the perceptions and expectations of the group.