Spoiler: showbefore I go any further, for clarity I'm not suggesting this has been their plan all along, just that it seemed as if they're retconning in a way to say yeah this who these people are, enjoy!
that said, that opening scene didn't necessarily have to be the crystarium-- just reminiscent architecture. And Ardbert co. were only able to tip the balance in favor of light because they act on its behalf. presumably they all have/had the echo