To be fair, the "reason/explanation" that Gu Changge had Xian'er think of about the events of him taking her dao bone when they were merely kids is likely the actual truth.
Pre-system Gu Changge & Ye Mingkong Past Life Gu Changge version are both dominated or heavily influenced by their demonic heart/nature, instead of their actual self. Likely, the demonic influence become nonexistent when the system took effect, allowing our current protagonist to finally gain control.