Hmm... it seems that Fern spoke off-the-cuff, so I'm not sure how reliable the information is, but I'll take him at his word. So, there are other Echo-users in the Scions besides Minfilia and the main character.

I know for sure that, before the massacre at the Waking Sands, there were at least three Scions who were former Pathwalkers. One of them, a miqo'te mage, was particularly frustrated that none of the other Scions remembered who they were. It's frustrating for her because she and her two companions were among the most senior of the Pathwalkers.

But that trio of Pathwalkers were killed when Livia raided the Waking Sands.

Since then, I haven't come across other Scions who appear to possess the Echo. Hoary Boulder and Coultenet are the only two regular Scions you see at the Rising Stones, and they don't appear to be Echo-users. Some Scions are still based in the Waking Sands, but they don't strike me as Echo-users either.

So, who are the "most of them" who have the Echo? Hmmm.....