Biodegradable plastic is as good as fully compostable (e.g. according to EN 13432).
Moreover, biodegradability is only a part of Compostability, where, among other aspects, eco-toxicity and physical disintegration must also be tested before a material can be called compostable.
It is also good to know that there are compostable materials (in accordance with EN13432) which are not 100% renewable (e.g. PLA blend with Ecoflex).