Siuan's statement does not mean that a weak Aes Sedai could not. It's an indication of that, but it might also mean a number of other things. She could mean that a weak Aes Sedai would not be able to use it at all (like the Choedan Kal, although I believe this is unlikely). Also, saying "a weak novice could use it to crumple the walls" might reduce the mysterious reputation of the Aes Sedai. Indicating that only an Aes Sedai could use such a powerful artefact makes them seem more powerful than they are, and as such, it seems like something they would teach novices, since some of those who do learn might leave the tower before attaining the shawl. Or there might be another reason for her wording altogether.

My point is that Siuan claims a strong sister could do something - she does not say that a weak could not. It's an indication of that, but that sort of "truth" is exactly what Aes Sedai are famous for.



Does Elza draw on the full potential of Callandor? We don't know.

Also, it's quite possible that since Callandor is flawed, it doesn't work like other sa'angreal. I've always assumed that you could draw a specific amount of power from Callandor safely - and then draw an infinite amount more from it, but since it's not safe, you'd die before you could unleash something even close to the strength of the Choedan Kal. If Rand used too much Power with Callandor - but not enough to burn himself out - that might be why it shines brighter when he uses it.