If the seatpost is aluminum and the frame is steel, it might be one with the frame by now. Steel on steel you might have a chance with a robust pulling device. DOT 3 brake fluid is an excellent iron oxide penetrant but it will ruin paint.
Try a heat gun on the seat tube too, but only if the post is steel. If the post is aluminum, try dry ice down the tube.
If the bottom of the seatpost is abutting the BB shell, it'll be very hard to hook it.
If like most straight seat posts the top is flared in a little, that'll make the starfangled nut and quill stem ideas a little more difficult. Removing that flare would be a headache.
At least you found it in early days. I don't know how many times I've gone to adjust the saddle height after hours of labor only to find a show-stopper like this. Now I typically check it with a quick twist and would have missed this one.