This is going off-topic, but I'm curious. Does this mean I can take a GPL project, modify it, then sell the binaries+source for $$$ and refuse to release my modifications for free?
Roughly speaking, with GPL binaries can cost someone money but source code is always free of charge.
That is not required by the GPL, you only have to distribute source to people you distribute binaries to. Those people can decide to redistribute it (or not) to anyone (including upstream) of course, and they are bound by the same terms as you are.