So the typing for Mega Sceptile is that? I thought it would just be pure Grass.
Yep, but only for it's Mega Evolution. It goes back to pure grass when it transforms back to it's original state. As far as I'm aware, Mega Swampert is still keeping it's Water/Ground dual type. Mega Blaziken will probably be the same as well, unless it gets a new ability when it Mega Evolves.
There's a big announcement on June 15th which reveals a load of info for the Gen 3 remakes too. So that'll be interesting when that comes along.