I can't imagine why a cylinder bore would have issues with slugs, perhaps the instructions were referencing a choked model?
Or they are worried about nose bleeds from people shooting heavy slugs with their strong hand thumb draped over the wrist of the stock....
I could see a kydex holster with options to belt mount, sling or strap to a pack being pretty handy for people looking for a sheath.
The cylinder bore had me assuming slugs were fine as well. Hopefully your hypothesis is correct about a different choked model.
And yes, x1000 on the kydex holster idea.