The so-called "immortals cannot save" was a joke to Mo Xiu.
Before Mo Ruyi and others were also poisoned, weren't they still easily solved by Mo Xiu?
However, his solution to the poison on Mo Ruyi and others' bodies was to use the Myriad Wood Spirit Art to forcibly purify the poison in the four people's bodies with the strength of the True Energy, not a deliberately formulated antidote.
Now, he could also use the Myriad Wood Spirit Art to detoxify Yan Capital and the people in the villages and towns around Yan Capital one by one, but that was too troublesome, and he didn't have that leisure time.
For him, if he really wanted to use this kind of 'native method' to help people detoxify, it was better not to save one at all.