Without saying its "wrong" (different strokes, and all that) and ignoring the specifics of snowboarding, you should think of the best stance being any stance that will allow you to shift your weight/balance quickly with stability.
Bottom to top, the available joints are ankles, knees, waist, back, neck.
Run up to a friend(s) and try to shove them a couple of feet. Watch how they try to maintain their balance. Most people (who don't fall) will probably bend their knees and lower their center of gravity to reduce the moment of inertia acting to tip them over...and THEN bend their waist or torso after they have a relatively stable lower body.
Almost no one (who is sober) will stumble side-to-side stiff legged without bending their knees much. Locking one of your joints into a fixed position (like keeping your legs straight) eliminates one of the tools your body can adjust to right itself, and definitely makes staying upright more difficult.
Its not impossible to ride a snowboard with relatively straight legs, but it requires much finer skill and speed with balancing by a rider
With balancing in general , multiple, quick, small adjustments are better than infrequent, huge adjustments. If you are only using your upper torso to move your body, then your adjustments will be bigger.