Bad grammer. Understood.
...it is found in the (originally oral) Mishna.
Now I ask you, "Just what stuff that is WRITTEN down WASN'T 'originally oral'?"
I guess the conclusion a reasonable person can come up with is the either Rome messed up by NOT including it when it compiled the Jewish writings into the Old Testement, or the Jews, themselves, failed to hold it in that high of an opinion.
https://en.wikipedia.org/wiki/Mishnah
(With or without the final "h")