@6:05 "Join UV's" is no longer an option in 2.78. It is now "transfer UV's" and furthermore, the geometry bus be exact matches now in order for control-L-transfer-UV's to even work right. Never had this problem before. Very confused!
I'm pretty sure geometry has always needed to be identical for transferring UV's to work. It'd have to be super robust for it not to require identical geo..
My geometry is not the same on all the toes. I did line up the UV's on top of each other and then snap the borders like you did, but is the non uniformity going to cause problems when I paint the textures?