When brandon says sections refer to Elantris, I think he's talking about the mention of Aona (Aon, get it?) and Skai dieing as a result of Rayse's visit to Sel.
Also, I'd like to make a distinction: Ruin and Preservation ARE shards. Ati and Leras were the people who held them. They BECAME Ruin and Preservation once they took up the shards, so technically now Sazed could be call both Ruin and Preservation.
Since Ati and Leras both gave their names to the metals that are their body (Lerasium and Atium), what can we make of Aona's name, which is surely the root of Aon? Are Elantris shards bound in language somehow? And is there any 'Skai' sounding word in the book?
I don't think it's Galladon. Roshar is too far away for them to cover up their Elantrianness with illusion. Also, going to Teod does not make Raoden appear less Elantrian, so I think their appearance is not tied to the city once they have fully transformed... Clubs is every bit as grumpy as Galladon, and yet they are not considered the same person.
I think what fRR post means, is that those three ARE from another world, but not characters we know...