Oops, my mistake - I thought mega evolution was first revealed in June 2013 Coro Coro, but then went into further detail at E3, but I just went back to look and you're right. I probably got confused with E3 2014 where they revealed the Hoenn starter megas and Sableye. Still hoping they talk about it during E3 though!
The reason I don't count Sylveon is because it literally looks like it was designed and made just to promote the fairy type. It doesn't look that similar to any other of the Eevee-lutions, and the fact it's the only cross-gen evolution in that region doesn't help it either. And tbh half the time I forget Hoenn even has gross-gen evolution because it literally had like two, but at least they got expended on in Gen 4 a little with Roserade/Budew, Gallade, Probopass, Dusknoir, Chingling and Frosslass.