Part of working with the player is ensuring that the scene was described well enough for them to make a proper choice.
Often, when a player responds with an action the DM thinks "doesn't make sense" it is directly because the DM didn't describe/frame the scene as completely/well as they thought they did. So I agree with you, and I don't think you're actually disagreeing with me (maybe I didn't frame describe my response as well as I thought I did and need to make sure we're on the same page).
Let's take the OP. Clearly Mercer thought a 35 foot tall monster was kaiju level big and wouldn't be subject to stuff like sentinel. But to many, 35' isn't THAT big and it clearly should be (subject to sentinel). Had the monster been 300+ feet tall, we probably wouldn't even blink at his ruling - that's terrain level big. What we have is a disconnect in framing/description between player and DM need to be on the same page here.