Considering that Firefly/Fireflight is using a harrier-type jet mode, I don't think it was any surprise that a redeco as Slingshot was going to show up in one market... or both eventually.

By releasing Firefly/Fireflight first, people are too excited with the new mould to notice... but if Slingshot was released first or at the same time as Firefly, people would complain that it is the wrong sort of jet mode and that Hasbro/TT are just being cheap for doing it.