This video surprised me a bit. Please forgive my presumption as I am not a frame builder, but I agree with Norris that cutting off the head tube through the lug, then reaming, filing or grinding off the part of the lug left on the top & down tubes would be better practice. Although the color of the hot tube may not be true on the video, it sure looks like it got hot enough to mess up the crystalline structure of the steel.
Michael Shiffer
EuroMeccanica, Inc.
114 Pearl Street
Mount Vernon, NY 10550
(914) 668-1300
euromeccanicany.com