Problematically, this fact is not specified anywhere on the Humble Bundle page or download screen. Prior to purchase or install, there is no indication that multiplayer is disabled in the DRM-free version.
If you try to access multiplayer, the button will be grayed out and cannot be clicked.
Team17, the developer confirmed on Twitter that only single player is available for the DRM-free version.
It should also be noted that Worms Reloaded multiplayer is not cross-platform on the PC platforms either. Whilst we might not expect Android and iPhone versions to be compatible, it is surprising that Windows, Mac and Linux users can only play with people using the same operating system.