This is crap. Its not true. These people could not read. So they could not have a phonetic language based on English. Its possible that a very few people who were just starting to read could speak like this for a while. But a language, no! Somebody is revising history.
I don’t think they said it universally used. A “secret language” isn’t something people spoke in everyday life. For many slaves, it may just have come down to a few words used when secrecy was important, something like “Pig Latin.” Slaves who did secretly learn to read developed the code and then passed the words on to others. Maybe “secret code” is more accurate than “secret language,” but that doesn’t generate headlines.
Not just that, but slaves did not interact with each other, they were isolated so they couldn’t come up with a common language.