They mean we can't get accurate numbers on the effects. Yes, you can try to figure out what the fuck someone dying of "wolf blood madness" means in a report from 1824, but what you don't know is how many deaths were unreported in the first place.
While I sympathize with the notion that we should hold big plastic accountable for all the trash, the solution is not to not reduce, reuse, recycle. All that crap is already there, we need to do something about it, just going "meh, recycling is just a conspiracy" is actively harmful