There are multiple and confusing definitions. You have biodegradable, you have compostable and you have oxo-degradable. The one that you were describing is really oxo-degradable: An additive goes in and creates a kind of stained glass. The additive is the lead in the stained glass, and as it decomposes, you do get the microplastics. However, there are technologies out there, and there are plastics out there that are fully compostable in either a home or an industrial recycling setting.
I think that was early days, but with technology today, my response is no, that is no longer truly the situation.