I’d be surprised to learn of any conductor who couldn’t play the piano (yes, I was surprised to see that Berlioz couldn’t, I thought all composers could), for several reasons. One, most of them graduated from a conservatory and some ability at piano is usually a requisite for graduation. When I was in college you had to demonstrate some piano skills just to concentrate in music, and that wasn’t a conservatory.
Second, for the Europeans at least, most conductors came up through opera houses as assistants — and playing piano to rehearse the singers would have been part of their job. Toscanini was a rare exception — I think, at least, that he was never an assistant conductor but moved straight from cellist to conductor. But he could play the piano too — there are lots of reminiscences of his doing so. (And remember, his story is that the night he first conducted in Buenos Aires, he was late because he was at the hotel rehearsing one of the sopranos in the chorus ….)