Right, I got the bleeder out. 3 kettles of boiling water, left foot in the downtube/seattube junction, right hand on the back of the saddle twisting, left hand on the nose, pulling. I won about a millimeter of seatpost per quarter turn, then I had to return it to center for another twist n pull. slosh of boiling water every 3 or 4 goes. Did that for about 3 1/2 inches before it freed itself, when the clouds parted and choirs of angels sang beautiful harmonies.
Runs out it's an ally post with a carbon skin. The carbon skin is less than a quarter of a mm.
My Thomson wouldn't go in the frame without some sanding away of hard white stuff. Now the Thomson and the carbon skin post fit with exactly the same snugness.
So in this case, I don't buy the swelling idea as the main cause. Galvanic or not, there was definitely some corrosion down there.