Bottle on Slark? Does Slark typically go mid?
Slark can go mid. Slark can go anywhere. Slark can build anything. And you don't have to be mid to pick up a bottle. Sometimes your mid doesn't need bottle and sometimes they don't even know how to rune control, in which case the runes are going to waste on a useless mid when you can be using it to fuel your aggression.
His free roaming speed allows quick trips to and from the fountain so even if he's not getting priority on runes, it's allowing him to stay in lane just a bit longer to get kills or farm.