Cool, didn't know that. Now just hope they don't make us sick or we're really screwed. With all the microplastics in our bodies they're going to be everywhere.
Toxicity isn't as simple as "toxic = toxic + toxic." While some byproducts of plastic breakdown are toxic, the bacteria are further dissolving those as well, going until they get glucose, as they wouldn't be able to eat it if that wasn't the end product. There are probably still some toxic byproducts that get excreted rather than broken down, but plastic breakdown already releases toxins under normal conditions, so that's already a problem we're going to have to tackle. If these bacteria can get past the first issue of breaking it down in the first place, then that's a net positive.
It's a part of a potential solution, but right now if you dump a bunch of plastivores in a trash pit instead of a bunch of plastic in a hole that won't break down from a thousand years you get a toxic slurry capable of entering groundwater supplies.
Of course, micro plastics are also doing that, so pick your poison I suppose.