I found the writer of the article (and the writer of the first comment) confused about the GPL. The author writes:
"Copyleft (a play on the word copyright) is the key factor that differentiates the various open software licenses. Copyleft is a license requirement that governs the distribution of modifications to the original open source software. If a license contains a strong copyleft provision, anyone who modifies the source code and distributes it to the public must license the modifications back to the public under the same terms as the original software. This means that you must give up private ownership of any changes you make to copyleft software, unless you elect not to make the modified software available publicly."
The writer of the derivation has copyright to the modifications he or she makes to the original code. This copyright is a 'private ownership" of the work created and the control of distribution copyright law gives the author is the legal basis of the GPL. The GPL could not work if this "private ownership" didn't exist.
Copyleft does not control the distribution of modifications of software -- the modifications themselves can be distributed under any license. The confusion the author has comes from conflating the idea of modifications and derivation. The derivation is a combination of the original work and the modifications. Nothing in the GPl license gives the person making modifications any copyright control over the original code -- that means that the second author has no right to distribute the original code except in the manner the original license allows.
The commentor also make this confusion here:
"The offeree who accepts a non-negotiable open source
license offer must not just grant a waiver of rights to
the offeror but must also offer a waiver to all third
parties of his granted copyright monopoly -- the
offeree's "rights against the world" -- rights granted
by the Copyright Act. It's one thing to waive a legal
right in privity but an entirely different thing to
waive a right against the general public."
There is nothing in the GPL under which a person who creates a derivative work by creating modifications has to make the either the derivation or modification available to the original author or to the general public. The only obligation occurs when the derivation is distributed and only to the persons receiving the distribution. If the second author chooses to they can distribute only their modifications and as the author and copyright holder can choose any license they wish. If they choose to also distribute the first authors copyrighted code (which always happens when distributing a derivation) they must abide by the license the first author has chosen.