It's worth remembering that plastic doesn't start out as plastic - they start out has hydrocarbons which are linked together to form long chain molecules we know as "plastic". This, if the article is correct, implies that the polyethylene they are working with is broken down from the molecular chains into the C2H4 basic ethylene, or into short chains which can be stabilized into a surfactant which naturally decomposes into plain ethylene and might be used for the normal industrial synthesis of ethylene based compounds (like detergents and antifreeze, among others). The plastic, as a macro(/micro/nano) particle, would be gone and replaced with the target chemical (again, if the process is as they describe and complete). Whether the resulting surfactant is degradable is not addressed. Again, it's hydrogen and carbon...there's a lot of ways that can go - good and bad.

My chemistry is nowhere near good enough to evaluate the claim. And scientists do get it wrong but I think he'd likely know. And it makes sense. They're not using it to make an abrasive soap, they're using it to make a surfactant. Which is liquid, not solid AFAIK.

That's not to say the product won't still be problematic, but possibly no more problematic than existing surfactants used to make soap.

I don't know, and I think your general concern about releasing things that were once plastics into the water supply is reasonable. But the plastic is going to end up polluting the earth in one way or another, in one form or another. At least they're using up the old stuff not generating any new.