Removing an internal headset before pulling the fork?
This is the compression ring. It does the same thing as the silver one on the Colnago in your picture.

Your fork is stuck because it's wedged in too tight.
You need to whack the top of the steerer hard with something that won't hurt it, and you need to have the fork off the ground while you're doing that. Hit it as though you're trying to send it flying out of the bike. Support the frame under the top tube or down tube with your other hand. You're trying to move the steerer inside the compression ring. Using a rubber mallet or dead blow on the end of a steerer will usually hurt the tool. A block of wood is a good idea. You can tape it on if you run out of hands.
Digging out the ring by putting a screwdriver in the slot can also work, but could also tear it up if it's really tight. There's some advantage to doing it this way if you can because then you're not hammering on your bearings. But it usually doesn't work, and upper headset bearings have an easy life anyway.