Definitely still worth trying to heat it because - I think - expansion is not all down to the material. Radii of the tubes and thickness of the material make a difference too (I think??)
Try with a kettle of boiling water if you don't have a heatgun. I've used that successfully to remove crown races a few times.
Also remove the saddle from the seatpost and try to clamp something longer to give you more leverage for twisting/turning.