Well it is, if they fail to take into account caltrops working on golems, oozes, fire elementals, etc., etc.. Basically the a lot of this comes down to the fact that D&D is not written to be a simulation. It's a game, and it's rules don't always take verisimilitude into account.
Simply put, if you're going to write rules with the assumption that they do what they say they do in all circumstances, put a disclaimer on your game saying "hey, sometimes rules don't make sense in the logical world we live in. D&D isn't that world.".
In the end of course, the DM is certainly free to change or ignore any rule that makes the campaign less fun. Perhaps Mr. Mercer felt allowing a puny Feat to reduce the challenge of his encounter significantly was, ultimately, less fun for his game. The problem I have is less that he decided to do this, it's more the when and the how.
Doing that when the player is thinking to themselves, well my character has this ability, so I'll do this in the combat isn't great.
And it's made less great when, instead of explaining why it doesn't work, the DM says "now you, player, who have nothing to go on but your own potentially meager knowledge of fighting giant monsters, and have no information other than some vague flavor text, justify your ability's existence!"
Even if the player had come up with a defense, it's the player convincing the DM, and has nothing to do with their character or their abilities. "Ability to persuade the DM out of character" shouldn't be a necessary skill to play D&D.