Google has removed the datePublished “recommended” property from the fact check structured data documentation. Google said, “Currently, date information isn’t used in the Fact Check rich result.”
The new page removed the section for datePublished, here is where it showed this in the old docs:
What is interesting is that it is still shown in the example code in those help docs.
In any event, Google currently does not use date information for Fact Check rich result.
Forum discussion at Twitter.