It might be more helpful to redefine signature abilities as meaning "in the context of this generation, this ability is only found in this one Pokémon or evolutionary line" rather than "this ability is the exclusive property of this Pokémon, forever." Bulbapedia has a list of signature abilities which shows you how an ability can lose or even gain signature status as Pokémon are added or cut between generations.
From a marketing standpoint, a signature ability can be a way to make a Pokémon seem more interesting when it's first introduced, but once a Pokémon isn't new anymore, there isn't much of an incentive to maintain that uniqueness. So it makes sense for them to let more Pokémon have the ability if they want to reuse the idea.
As for Varoom having Slow Start, it's probably just a little joke about car engines that start up slowly. In real life, vintage and luxury cars can be highly valued and sought after despite often running worse and requiring a lot more maintenance than standard models. Likewise, a Varoom with its hidden ability might be desirable just because it's rare, even though it'll be harder to actually use it in battle. The good news is that if you do put in the work to take care of it, it'll get Filter when it evolves, which is much more useful.
