No...you're ignoring free-air vs sealed driver resonances.
I know of no headphone that uses the same exact driver to make both a closed and open variant of the same. Even if they did, the resonant frequency (absent EQ) would be different, thus making them different headphones.
I don't understand the point of this thread...
Yes, my bad. I can remove the wooden cups from my Denon D5K, which would probably be the closest thing to a legitimate test from a non-scientific consumer option. The various, though subtle, FR differences found in recent Massdrop Fostex TH-X00 headphones should have been a reminder to me that cups both opened and closed, can make a difference in FR. I was trying too hard to conjure up a situation where a combination of room environment and open headphones could be made to sound identical to a closed headphone.