That's the problem with assuming that Kirk never left the Nexus: it raises the question of how we know that Picard did.
It would have been better (IMO) to address this obvious issue out in the open in the movie itself. Even just leaving it as an open question would have been good. Just acknowledge it. Picard could have had a conversation with Guinan on the ship that presumably takes them back to Earth at the end of the film:
Picard: "So is all of this really happening, or--"
Guinan: "--or are you still in the Nexus? Good question, Captain."
Guinan smiles and walks away. Picard looks out of the window in deep thought. Cut to exterior shot of Picard in window. A reflection of the Veridian sun is on glass. Zoom out as ships go to warp. Fade out. End credits.
and the top keeps spinning on the table in his ready room, too...