correct, 2000+ SEs did not come equipped with a rear sway bar, in fact not only is the suspension beefier looking, but it is also a different design and re configures where the vehicles weight is distributed.
Im sure that a swaybar could be added, but i havent looked into it at all
Also, not to cast doubt on your powers of observation, but I noted that on my wife'* 2004 LeSabre, the rear sway bar is mounted forward of the rear supension parts, as opposed to behind them (like on my 199. When I first crouched under the rear of the '04, I did not immediately see the swaybar...
When I upgraded my front and rear bars, I gave the rear one to someone I knew with a SE that was not equipped from the factory with one. FWIW, he was thrilled at the difference in performance once he had it installed.