Can't use Slugs?

I'm honestly scratching my head over here. Im not familiar with that model(835) but what could the reasoning be?

Does Mossberg offer a specific slug gun? I was under the assumption that it was your average shotgun and could handle anything its chambered for.

I know for a fact my Uncle used to shoot slugs out of his 500 b4 he sold it, I wish he still had it so I could call him up and he could check it.