The cure for spirit poison is expensive and has to be applied quickly, but it is possible to cure it. This has reduced the threat of spirit poison to corpse cultivators. After all, it is the only poison that can affect them, so they always have an antidote prepared for it.
This is why the people from the Black Bones sect didn't take the threat seriously and could even challenge the Purple Flower sect without fear. They have very little to lose if they do.
The elders from the Blood Skull sect also didn't take the threat seriously even though the normal poisons of the Purple Flower sect can affect them.
No one needs to bring out the spirit poison to threaten the people from the Blood Skull sect because there are less valuable and more varied poisons to affect them.
