I am not from Intel, but I would guess that your particular situation is not the TV IR, but rather something else like HDMI CEC you mention. One way to temporarily disable CIR for testing might be to put some thick/heavy dark tape (or thick cardboard?) over that front right portion of the NUC case. If the problem remains, maybe verify it is HDMI by temporarily unplugging the HDMI. I believe that the CEC settings are usually in the TV set menu - I don't know the Sony but maybe it has some special name like Bravia Sync or Link. I am not sure but I think on some TVs you can change that setting differently on each HDMI port, so you could disable it on the port for the NUC but keep it enabled for something else like receiver. But these are all guesses - one more possibility is that the reason it starts up is something else such as certain network activity.
Your situation caught my attention because my NUC power up when I press the PAUSE button on my remote, it's really frustrating like you said that this cannot be disabled in the NUC.