Andyomon00
Seer of Doom
With the same logic, couldn't you also argue that the reason they showed a Butterfree that wasn't its Alola variant was because they didn't want to spoil the Alola variants?
No, you really couldn't. A lot of pokemon from different generations appeared in trailers for sun and moon, but those specific pokemon were never shown in anything for the game up till this point. Butterfree shows up in the trailer looking the exact same as it did in all previous gens, but the regional variants have only in their region specific form.