Mark's comments in the live stream and his follow-up Instagram posts indicate that the Pretenders were a fairly late addition to the line-up once the initial intended figures fell out.

I wonder if the "Energon Monster" tag was supposed to be the intended figures like the Fossilsers - and designed to tie in with the energon weapon theme they have for their figures and that they couldn't make them work as intended so went with Pretenders.

On one hand I'm much more glad we got these pretty amazing Pretenders, but on the other hand I'm wondering what the Energon Monsters were supposed to be and if that then explained some extraneous accessories like Blitzwing's hand turrets.