I think none of the dishes have official designation as Malaysia's national dish, not even Nasi Lemak, even though it is often claimed to be one. Also that if Satay is Indonesian it would not preclude it from being a Malaysian national dish since both people are closely related. People in Singapore for example would claim some popular Malaysian dishes as their national dish (Hainan Chicken Rice, Laksa, etc.), and their origins are also disputed.   I think the point is that there are many sources for the claim that Satay is a national dish in Malaysia (you can find some the sources given above, more than you can find for Laksa), so there is a consensus that it is considered one of the national dishes of Malaysia.