No, it's not true; the behaviour in any standard build of rockbox is identical no matter what you plug the ipod into: it will draw only 100mA, and thus will charge very slowly if at all. With the patch from FS#8802 and the usb charging option turned on, it will draw 500mA from whatever it is plugged into. We have no code to detect what it's plugged into if it's not a USB host.
It's generally possible to charge the ipod anyway even at 100mA as long as its not being used: no disk activity, no playback, no backlight - it will charge at ~50mA which simply takes a very long time (at least overnight). If you are actively using the device to copy files to/from it via USB then it will discharge as 100mA is not enough to power the disk, but obviously you aren't doing file transfers while plugged into an AC adapter instead of a computer
With the original firmware (or the patch) it should fully charge in only an hour or two.
Edit: correction, it will charge the same no matter what *USB* source you plug it into: things which power the 12V FireWire power pins charge normally if you have a model that still has those pins connected.