Let's face it, there is nothing "Hextech" in this skin, not by today's standards. One can tell just by comparing it to newer instance of the skin serie.
I know, Anivia, Janna and Singed also possess Hextech skins that are similar to Sion's, but they are aging champions that will eventually be updated and their Hextech Skins too, to match today's definition of "Hextech". Galio had a similar Hextech aesthetic and became like the newer instances after his rework, and even came out with new VFX and SFX.
But Sion already got updated three years ago and his Hextech Skin aesthetic didn't follow up. At best it's a Battlecast/Resistance-ish skin, but certainly not a Hextech.