While true, bamboo rayon also isn’t the best for human health or the environment, so it really is a pick your poison kind of deal. Here’s a short write up from Patagonia about why they don’t use it in their products, and of course a lot of Patagonia’s things are polyester or polyester blends. (The terms viscose and rayon are generally interchangeable.)
It doesn’t seem obvious to me which is worse between wearing polyester and bamboo rayon, health wise, but I do personally find rayon much more comfy.
...and of course the comfiest things of all are tri blend t shirts, which contain a mix of both polyester and rayon. you can multi track drift poisons!