Yeah. I'd like to know how that happened. I suspect the DM didn't fight very nasty--for one thing, Vlaakith is down to only 5th level slots right now, and yet she isn't Shapechanged into an ancient dragon, she's just in her regular old lich form apparently. Also, the fact that multiple PCs are low on HP instead of a couple of them being dead is suggestive that the monsters were not focusing their fire. (As mentioned previously, 2 adult red dragons can do about 200 points of damage per turn to a single target using legendary actions, around 150 HP of damage once to-hit is accounted for, and that's not even counting the spectral knights and spellcasters. The PCs can blow a bunch of actions on casting Anti-Magic Field (which might get Counterspelled) and grappling Vlaakith into it, but meanwhile the bad guys can simply annihilate whichever PC is doing the grappling. I suspect this did not happen.)Actually, according to the OP's update, the PCs' victory is all but a foregone conclusion.
It's okay though, to a certain extent. Even when something is billed as a deadly fight, it's emotionally difficult to pull the trigger on your friends. After all, Vlaakith doesn't really get excited if she TPKs the player characters--but there are actual real live human beings (the players) who will get excited if their characters triumph over Vlaakith. I'm not saying that the DM was wrong to be less than 100% maximum nasty--just that, from the previous thread, I expected a high level of nastiness from Vlaakith which may not have actually manifested in the real fight.
And I might be a little bit disappointed, as a player, with that lack of nastiness--but not enough to let it mess up a friendship.