The website shows Belly Drum still has the same message. I think it's just that the "maximized its Attack" message is a direct result of using Belly Drum and the Mirror Herb is just giving it the +6 boost which gets described as drastically for any boost above +2 like with Steam Engine.
So it's number three indeed
Also, is it already possible to do some math and estimate the power of terastralizing? We have seen tera brelloom using bullet seed against blissey and doing a 4-hit KO