Riffzor wrote:... although on first glance it looks as if the special rule only applies to the Biker mobs since it's under their 'Special Rules' listing in the army list.
That's a side-effect of the funky way the earlier '2nd edition-esque' codexes were written. Where a particular item or rule most commonly belongs to a particular unit, they put the rules for it in that unit's describo-page, and refer everyone else with access to that item/rule back to that page.
Rather awkward way of doing it, which they seem to be slowly abandonining in the newer codexes.