I'm not familiar with Ranger Knight from the game - it is indeed one of Larian's creations, but what I can say is this: if it's a choice that you make, as the player, outside the single discrete choice of selecting 'ranger' as the class level you want to take, and the Heavy Armour proficiency is contained within that separate discrete choice (similar to cleric multiclass choosing cleric, and then choosing a domain immediately as a discrete choice), then you absolutely should be getting it.
