consider the timeline:
she gained sentience (became a spirit)
she met & learnt garbage techniques
she tried to seduce a nun then got locked away for 300 years
during 300 years imprisonment, she cultivated buddhist techniques and open her nose acupoint
so she could transform even before she reached the 1st realm - a fact that is actually beyond the chat group's common sense.
this explanation might seem unsatisfying, but know that she is an anomaly whose fate/karma is linked to song shuhang