That's a very blanket statement and a very common theme for answers given by fans in defense of a plothole or inconsistency or how the author had it planned out form the beginning.

It's also utter nonsense Dan, Butcher either makes a character that seems cool and forms a story around him or makes a plot and adds the needed characters or more likely, like most writers, both in a feedback process. But the characters always have an initial prototype form and change. He didn't have the whole arc plotted out by character since Stormfront. He's added, changed, developed. It is very unlikely for instance that the moment Susan or Michael appears in the books Butcher already knew Susan would have Maggie and be sacrfiiced, or that Michael had two buddies name Sansa and Shiro and an archenemy Nicodemus and a daughter who would end up Harry's apprentice and the winter lady. You're implying he did, and maybe some of it he did, but far more likely his creative process has continued to add stuff to these characters.