How did you change it from the original design, then? All I see in your account is that it remained as you had made it.
This is what I meant by having a look at whence those suppositions are coming.
The problem is less that you supposed wrong than that you gave the players no chance to suppose right.
I think you may need to reread his post.
There are three different encounters that were possible in the dungeon the PCs were in.
1) A wandering patrol of constructs that the PCs had the opportunity to engage earlier, but instead avoided and left free to roam the dungeon.
2) Several constructs within the room that would activate when a second door was touched - something canny adventurers might anticipate, but which there was no absolute way to determine on their own.
3) An escort sent after them when they enter the room and triggered the illusion, which they knew would be heading their way.
You seem to believe that the players had no agency here - that all possibilities led to having to fight all three encounters at once. That clearly isn't true. Even if you aren't saying Bullgrit intended for the fights to converge, but left it too open to random variables the PCs had no control over - that, too, is wrong.
The PCs could have engaged the patrol on its own, for example. They made the decision to leave it to wander.
The PCs were also aware an 'escort' was incoming. They could have tried to finish the patrol when it reached them, rather than made the attempt to flee (and thus activated the statues and run into the 'escort'.)
Yes, the original scenario allowed for the potential for easy fights to 'merge' into a TPK. But it also allowed for many other outcomes - this wasn't a foreordained result. It was the result of both the PCs' decisions and bad luck.
Could the players have predicted how everything would come together? Not necessarily. But full knowledge of the entire mechanics of the dungeon complex would likely have removed a good deal of the fun of the game. They still did have decisions they were able to make, with the potential to foresee possible outcomes. In the end, sometimes the odds do end up stacked against you - in this case, not deliberately, but through unfortunate choices and happenstance.
That doesn't seem a fault to lay at the DM's feet, unless one believes part of the DMs responsibility is to ensure the PCs never are in any sort of danger at all - which might suit some games, but certainly not all.