As required incidentally by the full terms of the GPL.
Not true. Read the GPL again.
From the GPL:
By contrast, the GNU General Public License is intended to guarantee your freedom to share and change free software--to make sure the software is free for all its users.
and:
that you can change the software or use pieces of it in new free programs
and most importantly:
We protect your rights with two steps: (1) copyright the software, and (2) offer you this license which gives you legal permission to copy, distribute and/or modify the software.
To comply with the GPL, there is no obligation to publicise what the product is at all. There is, however, an obligation to acknowledge the original author. That doesn't mean saying that it's "wordpress" anywhere on the website.
To insist that "wordpress" must be publicised is contrary to the spirit of what GPL is.