Not sure if it was still the case but didn't google uses to list the extraction sources? I guess the work that was done with google squared also fed into these boxes? I get the impression then that they do use more than one source if possible.

It is a very hard one as the vast majority of the time the text features observed would indicate a date of birth/ death. I guess if the date is not in the metadata/ sidebar on Wikipedia they could flag it and hopefully find another source to correlate with.