Seen a bunch of pros do that, technically you don't have enough mana to use it at 6 with gale and a decent amount of wards if you don't go arcane. A lot of the sidelane/farming venos seem to go treads instead and just skip ult, which imo is a retarded build. You should go for arcane and ult asap, lvl 6 ult is brutal as hell especially if you got xp advantage on supports but by 9 a lot of supports will be high enough they won't mind it as much until 11. You don't gain that much from skilling something else at 6 either. That said it's not a demon thing, seen it several times it seems to be the accepted build, but everytime it seems completely shit if they ever get into a fight before 9(and you always do). Even at 6 you can solo kill anyone you gale if you have your ulti. Solo killing someone will probably give you a level, meaning it pays for itself if you use it right away to get a kill on a support or whatever, and Arcane boots are obviously useful for everyone even if not totally useful for you but eventually you'll replace boots by travel regardless and treads don't do shit unless you build full carry which most venos don't even if they get farm priority, they go like agh veil bkb blink or whatever and treads don't help you for shit with that setup since you're not gonna right click anyone.
LGD picked smart and played their lineup well, Liquid played meh. Not really a Demon loss, just the whole team did poor stuff.