I don't mention cedar; I just talk about logs, brush, and dead
wood with no species name (I checked!). I would not normally recommend cedar logs as a good choice for
hugelkultur, but if they are already pretty rotten, they may not have much of the fungi-inhibiting compounds left. Now, whether cedar will actually inhibit the growth of plants, as opposed to many soil organisms, I don't know. You could try something small scale first.
I'm intrigued that cedar leaves are a good mulch for tomatoes. I wonder if they inhibit verticilium wilt and other fungal problems that tomatoes are prone to. That's worth checking out!