But they totally don't buff champs to sell skins.
Just to be clear, they dont do this for _all_ skins, mostly just the high Profile ones.
Take Zed and his legendary, he got nerfed on that patch yes, but it was a love tap Nerf, and was partially just a revert to the buff he record 4 patches earlier, which massively buffed Living Shadow's range.
Lux was steadily buffed to the point r/Lux was asking for nerfs, but didn't get those nerfs until a month after her Prestige skin.
Vayne got her Q cooldown reduction when ulting a patch before her prestige edition.
Pyke has just gotten a massive gold buff on his ult which coincides with forcing him back into the support role in a similar manner to Janna.
Even further, I don't think this is a bad business practice, skins are their income it makes sense to do this. Just don't pretend not to.