Note: Have you even read the article? They say they would make the source code available for official governmental institutions to analyze it if they wanted, they don't say anything about making it completely open-source!
Making the source code open doesn't help unless you can flash the devices, since you have no guarantee the source code is what's on the devices.
Plus, if the backdoors are in hardware (say the hardware AES implementation has a small key scheduling "bug" or something) , not software, source code wouldn't help.
Exactly. And any update feature, which is typically built into most networking products, could enable a backdoor to be installed at a later date. Could be clean now, but doesn't mean always!