This is misleading bullshit.
First, they mention the "phone" icon right after complaining that GV is "replacing the iPhone’s core mobile telephone functionality and Apple user interface with its own user interface for telephone calls, text messaging and voicemail" to try to give people the false impression that GV is stealing the phone icon, which an app can't even do.
Then, they try to claim that by cherry-picking calls and not letting the forwarded calls hit apple voicemail, they're "disabling" visual voicemail, which is just untrue. People could still call your iPhone's direct phone number, and visual voicemail would work juts fine.