Probably nothing is pressed on but probably pretty line to line fit and somewhat corroded or forced together onto a painted shaft.
I would spray that top section with a good penetrating oil of your choice but not WD-40. Then see if gentle taps up and down get some movement going and if so continue with the spray then tap routine for sometime.
If gentle taps don’t work then maybe get a bolt into the top of the shaft to allow more persuasion to be applied. May want to try and get the frame rigidly supported before hitting it too much, doubtful you can do much damage but th would get more force into the bracket.
Good luck and have patience.