I would be careful with that wording.
Bamboo fiber is plant-derived, but conventional PBAT is generally a compostable polymer rather than a fully biobased polymer. That means a PBAT-and-bamboo formulation may contain renewable material without being entirely plant-based.
A more accurate claim would be something like:
"Made with renewable bamboo fiber and compostable polymers."
If a brand wants to publish a specific biobased percentage, it should be based on an appropriate finished-product test rather than simply adding up ingredient weights.
"Contains 35% bamboo fiber by weight" and "35% biobased carbon" are not automatically the same statement.