>So does that mean, millions of WP sites with custom mods must relase release their code in some capacity?

Yes. WP plugin developers who charge for their plugins cannot reasonably attack people who bundle and resell/redistribute those plugins. GPLclub and other "resellers" of the plugins get a lot of crap from WP developers for pirating their work but it's very clear that this is not how it works.

EDIT: This is assuming you distribute/sell your plugin.