Look, I just want to say, I don't actually know what's canon. I just take stuff that I've heard and put it together in a way that makes sense to me. I'm saying this to make sure that this is a discussion, and NOT an argument.
That being said, that quote could be a metaphor for all we know. It may mean that the blood of Avarosa and Serylda live on inside Ashe and Sejuani, respectively. But the major point is this:
Sure, it's much harder to kill an Iceborn than a human. Iceborn have the power to control ice and change their appearance at will, which is something Lissandra does quite a bit. They are immune to the effects of aging, and won't die of old age.
However
One arrow to the heart, and an Iceborn is dead. If you bash in an Iceborn's skull with a club, dead Iceborn. An Iceborn could probably even bleed out, if they couldn't, like, seal the wound with flippin' ice or something. Granted, you'd probably never get close enough to an Iceborn to do injure them, due to the fact that they have ice magic and can "see" the world around them through heat vision (or lack-of-heat vision. They can sense ice). They might even be able to make themselves look like a loved one, making the would-be killer hesitate. We know that Avarosa was assassinated, but by Lissandra, one of the only people able to mask her presence from an Iceborn.
It's hard to kill an Iceborn, but it's pretty easy for an Iceborn to die. Or something like that.