There are twist beams withOUT sway bars, and with. Some of the ones with are not integral. The ones I know off the top of my head that have a separate sway bar with a twist beam axle are on some GM products I think (Cruze maybe?) and maybe Toyota.
My only point was to say that having or adding a sway bar on something with a twist beam rear axle isn't unheard of or anything. On the A1 cars, it was pretty much a necessity if you wanted to improve the handling. The A1 Jettas already had one (from the factory), as did some GTIs. External, mounted by bushings and links, pretty much EXACTLY like the add on ones for the B5s available now.
VAG just improved the basic idea by integrating one as well as on A2/A3/A4/B3/B4 cars adding the track correcting factor by canting the bushings. The B5s do not have that part, as they for whatever reason just stuck with the old straight style setup like the A1s.
By the way, I added one of the factory Jetta 18mm sway bars to the back of my homely '79 diesel Rabbit. Even with the rest of the suspension totally stock, it VASTLY improved the handling of that car. Which was good, because with only 48hp on tap, you really didn't want to have to slow down for turns if you knew you'd have to get back up to speed afterwards.