WordPress isn't even properly GPL-licensed. No license in the top-level app file [1] [2]. The included license file doesn't selectively attach itself to all relevant files, and not to source files that have a different license [3]. WordPress contains copyrighted material that's definitely not GPL-compatible [4], so even if the previous points are considered irrelevant, it remains questionable if the source can be distributed under the terms of the GPL at all [5].
IANAL and I'm not intimately familiar with the US legal system but I'm pretty sure the license applies to all code not explicitly covered by any other license, even if it's not added as a file header (otherwise all similar projects in which the license is merely distributed as the LICENSE file and referenced in the README would be unlicensed too). Term 0 actually only says a notice must be placed in the "program or other work" -- placing that notice in the readme seems sufficient if the readme is part of the program/work. The suggestion of how to apply the license is explicitly just a suggestion or recommendation, not a requirement.
The existence of non-GPL plugins without a proper exemption however might be problematic for re-distribution of the bundle in other GPL software. It could be argued that the plugin in question isn't part of the code covered by the GPL but then the official Wordpress distribution itself is obviously also not entirely GPL and can't be redistributed fully under the GPL.
For an example of partially GPL-compatible software re-distributed under GPL consider the program formerly known as IceWeasel: https://en.wikipedia.org/wiki/GNU_IceCat (a modified version of Firefox minus the proprietary branding).
It's interesting that the Broadway play opened on January 16, 1964, just 16 days after the starting date for automatic renewal of copyright set by the Act of 1992. Had it opened just a couple of weeks before, and not renewed¹, it'd be in the Public Domain by now.
¹ Only 15% of works were renewed after its initial 28 years of protection, which just goes to show the insanity of the current default duration.
The bigger problem is that the lyrics are probably in direct violation of copyright unless they were used with permission. They shouldn't be part of the GPL code unless the copyright holder has permitted that too.
As it stands there semm to be two readings from this: 1) the plugin is not part of the GPL code and de-facto unlicensed ("All rights reserved"), meaning the plugin can not be included in GPL redistributions of Wordpress; or 2) the plugin is part of the code and the code is therefore not eligible to be distributed under the GPL in the first place.
[1] https://github.com/WordPress/WordPress/blob/master/index.php
[2] https://www.gnu.org/licenses/gpl-howto.en.html
[3] https://github.com/WordPress/WordPress/blob/master/license.t...
[4] https://github.com/WordPress/WordPress/blob/master/wp-conten...
[5] https://www.gnu.org/licenses/gpl-2.0.html See term 7.