You are correct and validating my findings. Information on Stoeger Canada's site informed me that Beretta does not recommend slugs through anything tighter than their Improved Cylinder chokes. I thought wtf!? Slugs have ALWAYS been gtg through a modified choke, so I started down the information rabbit hole and found that at least with Beretta's Optima HP choke tubes, they are all tighter than our standard SAAMI specs. Not by much mind you, but tighter.
Edit: slugs pattern just fine out of their modified choke tube btw. Despite Beretta's recommendations, it's what I use, as it also patterns flightcontrol buckshot best.