That doesn't sound right to me, either. Let's use PLA as an example of a thing that is definitely plastic.

In the right conditions (hot aerobic compost, which is admittedly difficult to achieve), PLA rather quickly decomposes all the way back down to lactic acid.

Lactic acid is definitely not plastic. It's liquid, and is one of the primary products of the happy little microbes that I nurture in my pepper ferments at home.

Again, it's hard to pin the long-term properties of plastic (noun) down in absolutes.