What she means:
Something I love to think about is the fact that the Will Herondale we see in the TID epiloge and Nothing but Shadows (and will see in TLH) is the same Will Herondale we meet in Clockwork Angel. Meaning that this boy, this boy who tried to make everyone hate him because he believed he was cursed, this boy that had to carry a HUGE burden from the age of twelve and probably had accepted he would never be a husband or a father, this same boy is now both of those things. The stark contrast between the angsty, teenaged Will and the dorky father Will, who shows unconditional love to his children and is loved by them in return, is so important, because it highlights his journey and his development. And in the end, even though as a child Will had probably accepted he would die unloved and full of regrets, when his soul left his body it didn't linger on earth, since it had nothing more to ask of life.