So I think that the Sage's words from the painting are slightly mistranslated. Whenever it says "cannot", it's supposed to be "can not". As in "it's possibly not" rather than "it's definitely not".
So, I've reworded those statements to make it more clear:
RELIGIOUS ENLIGHTENMENT AND RIGHTEOUS DUTY MIGHT NOT PROVE BENEFICIAL OR HARMFUL TO ANYONE, BUT IT COULD ALSO PROVE BENEFICIAL OR HARMFUL TO EVERYONE.
EVEN IF ONE HAS OMNISCIENCE, IT CAN'T NECESSARILY BE CALLED ENLIGHTENMENT; AND EVEN IF ONE IS ENLIGHTENED, THAT DOESN'T MEAN THEY ARE OMNISCIENT...
This makes more sense in context with the next part, which says that even if you are both omniscient and enlightened, that doesn't make you a saint.