Much to the surprise of the researchers, the dialog agents started negotiating in an entirely new language when they were left to converse without human attention.
When placed in front of humans (online) who didn’t know who they were chatting with, an agent matched negotiation skills of humans along with English as fluent that the people were left unsuspecting about its existence.

Facebook’s AI Started Speaking Its Own Language And Negotiating Like Humans
Short Bytes: Researchers at Facebook trained their AI bot which they call ‘dialog agent’ to negotiate like humans and even converse in natural English language to
In my opinion, "Diverged from natural human language" is not the same as "speaking it's own language." From the paper it doesn't look like they did any kind of linguistic analysis on the "language" so it very well could have been just gibberish. Or as they refer to it "lower quality language."
The headline seems to be completely unsupported. It's to be expected that AIs will degrade the language, as the language uses multiple redundancies that aren't specifically needed in the limited scope of the AI's task.