Correct, they don't know how they work, but they do know how to use them.
really, once you get past that point - assuming Roshar hasn't been wiped out by them - the mistings and twinborn are all thats left, and shardbearers and about 4-5 radiants are not going to counter that.
So, even if Roshar catches up, they'll still be weaker.......
But we haven't even considered how the slaves on roshar would react, nor how the angry cities might suddenly turn on Elendel.
AND
Roshar has a certain stick.......