First off what the hell happened to that bike they are using in the video??? But to answer your question right about the 2min 50 sec area of the video you will see him reach under the fender with his left hand. That tells me he is using a nut on the underside. Iím sure that is what they are doing. Seat bolt holds at the 1 hole while bolts and nuts hold the lower attachment points. You could always press in nutserts if you wanted to go that way. Not an expert but pretty sure thatís whatís happening in the video. Good luck!!!
Currently bikeless but sticking around for knowledge.