Because this clause, as mentioned above, is a nominally ineffective clause.
The clause states, should a sect master "personally commit slaughter against a court official, the entire sect will be exterminated," which is too easily circumvented.
How many people does a sect master have under his command?
Why would he personally kill a court official?
Couldn't he send someone else to kill?
Would he be so foolish to personally do such a taboo thing?
Therefore, any sect master with any sense would not do such a thing.
Over time, officials stopped considering this possibility.
Initially, Prefect Song Yuncang didn't think of it either, but when things reached this point, he made a fool of himself in front of the entire city, cornered by Lin Xiaosu with one sentence.
