You can kind of argue the point with those on the list who are fairly obscure/extremely limited release (i.e. Rodney), or those who aren't technically Autobots (BM/BWR Silverbolt) and TFs like Impactor who have purple as a secondary colour, but even so, Skid-Z and Universe '03 Silverbolt are both undeniably mostly-purple, definitely Autobots, and preceded AoE Slug.

Slug is still a rarity, sure, but he's neither the first nor the only.