It's certainly true that everything in the game is negotiable by the DM- you are not bound by any rule or line of text in the game. What I find objectionable is the arbitrary nature of such rulings- the player is told what resources they can use to make a character. They select options based on what the mechanical text of those options says that they do.
Then the DM comes in and says after the fact, "oh but that doesn't make any sense to me, you know? And since I decide what makes sense in the game, it doesn't work now."
This is punitive to the player. If there was a time to ask "hey does this make sense to you?", that discussion should have come up long before the player attempts to use the tactic in a life or death battle at the table!
Now could Kaiju monsters have a trait that says "immune to effects that reduce their speed"? Certainly. And players should have a chance to know such things in advance. Certainly, their characters should know if Sentinel won't work on Kaiju, right?