That is a good question, and you're very right. There is a lot of confusion around the terminology.
To be perfectly clear, there are examples of fossil-based plastics—100% fossil-based—that are fully biodegradable, completely compostable. PBAT is one example. On the contrary, there is an example of a completely bio-based plastic that is not going to ever biodegrade, like biopolyethylene made by Braskem, so the word “bioplastic” should be avoided.
Plastic should be categorized into any of two categories. One, are these plastics certified compostable as a composite or as a plastic? This means zero microplastics and complete material circularity going into compost. Two, are they not degradable, meaning that they will pollute the environment unless there is effective recycling and reuse of those plastics?
I think “bioplastics” is terminology that should be avoided. We should call them either “compostable biocomposites” or “non-degradable plastics”.