I think any poison that leeches out of the wood would have done so long ago. I cut the 1" strips from a portable launching ramp I had made for my
dinghy many years ago. It had been outdoors exposed to the elements, and the lake, for many years. I needed to make a new one, so I salvaged what I could from the old. I could have used marine plywood but my budget did not allow that. I used what I had.
In any case, the treated wood is not being used in large expanses, such as a bulkhead. The relatively small number of 1" X 24" pieces are completely covered by the cedar ceiling.