Modern washers empty their water very rapidly and this requires a 2" drain. Yes, they also use a standpipe that must be trapped. When you used the laundry tub as a drain, you got away with it because the tub held the water until it drained through the small drain. Now, it is possible that this stacked washer does not drain as rapidly as standard washers, but it still needs a trapped standpipe. Washer drains hoses hang into the top of the standpipe so there is an air gap. You do not make a solid connection from the hose into the drain.