My guess is that it's just careless wording, or an outright error, in FS Publication 0039. It seems to not take joint returns into account. The maximum has been $5,000 per return since the program started in 2010. FS Publication 0039 is just informational. It's not the official rules. Form 8888 and its associated instructions are the official rules.
You would have to ask the Bureau of the Fiscal Service why their Q&A publication is wrong.
