Short-lived L-series from Saturn was based on an Opel platform, but still used the 4T40/45E transnission that GM OKed for towing in Cavalier, Sunfire, Alero, Malibu et al. There could still be other towabillity issues like suspension geometry, frame strength. Towing is not only about which automatic transmission is installed.
I see you say 5 speed auto. What transmission would that be? Opel Vectra didn't have one, jumped from the 4-speed to 6-speed in Vectra C, and GM didn't introduce the domestic 6-speed until 2008. A 5-speed Aisin was imported for some special applications like the 3.4 V6 in Equinox, Torrent, and Vue, but not the LW. The Aisin 5-speed was likely the "don't tow it" transmission you had to watch out for in otherwise towable GM cars that normally had 4T40 series automatics.
Tom Test
Itasca Spirit 29B