If I remember correctly, bamboo has a lot of silica, which is why the dropped leaves are shiny. I had a lot of bamboo in an area, and the ground under it was often dry when much of the other ground soaked in the
water. I think it causes a lot of runoff of the water, almost like placing a window on a patch of dirt, not much gets underneath it. Great for the bamboo, not so good for the other plants. So, I don't think it it poisonous, but my experience was I had to move the bamboo mulch, for anything to be successful underneath it. I put some squash, and lettuce near the bamboo, and the ones that were hard to get to did poorly b/c they were often dry and eventually got smoothered by the bamboo leaves. I gathered up the mulch and put it on places to smother things I didn't want. Seemed like a better option than carboard, since I didn't have to do any sourcing. Try it out, check under the mulch after a rain, not a deluge, i think you might notice it is pretty dry -- that was my experience anyway.