If your VIN says so
then I would guess it to be true. I watched the Lansing factories very closely. Odd things would come out for short periods of time. We always thought it was a strictly Grand Am/ Alero factory until one day a Malibu came out, then a Monte Carlo /Lumina who knows-chevy-crap-car, even once we swore they were making a mini-van in there. So it is entirely possible that your Bonneville was produced in MO. Especially true if you have a transition model year when they would re-tool the home factory (i.e.1991, 1995, 1999, 2003). Oddly, GM even mixed in different body types in the same factory The Craft Center in lansing made Sunfires, Cavaliers, and El Dorados at the same time).
GM really needs to rethink its production strategy to stay afloat. Soooo much waste.
1996 SSE Silver Toupe
2004 GXP Crimson
1992 Nissan Sentra
(Goners) 91 Grand Am LE; 91 Grand Prix SE2; 95 Grand Prix "Special" Edition