Just started Shadowbringer but I have a small question (spoiler)

[gilmorej]

I'm pretty sure that I remember the exarch stating pretty plainly that the reason we came here with our mortal bodies, all of our equipment, and can travel back and forth at will using aether trails from aetherites is because we were the specific and intended target of his teleportation spell, thus the spell was able to work correctly and in its entirety.

The reason the scions only got their souls teleported, and cannot travel back freely, is because they were not the target of the summon spell, they were accidental targets of a spell that was intended for you, and as such they have these rather unfortunate side-effects.

It doesn't have anything to do with him "practicing" or "mastering" the spell.. He just finally got ahold of his intended target, and so the spell was able to work fully as intended.