This is patently incorrect. Just remember the whole TiVo affair and reasons why GPLv3 was born. Source code availability does not guarantee ability to run it on the particular device.
The Software Freedom Conservancy thinks the GPLv2 guarantees the ability to modify existing GPLv2 software on a device, but does not guarantee the ability to still use the proprietary software running on top of that, and that the same applies with GPLv3. Reading the preamble of the GPLv2, I'm inclined to agree with them. Hasn't been tested in court yet though I think.
