Why can primals summon egi?

[Dotsusama]

Honestly I'd just say because it's a non-canon version of what they're capable of...kinda like HM Ultima.

That doesn't work because Ultima's Bane is a bard literally telling you "yeah I know it didn't go like that, but I had to make it sound good!" Whereas the EX Primals have actual questlines where you go kick their butts again because they were summoned with greater power.

That only gets stretched thin as an excuse when you go back for Primal Focus.