VFR traffic doesn't necessarily need radio, and is not affected by this groundstop.
As for lack of radio, this would only work if ATC blocked out your entire route from start to finish - which isn't practically ever done. Otherwise, they need to be able to maintain positive control over your flight since the airspace isn't reserved, which means conflicts are possible.
If you lose radio communication, it's an emergency and they'll move everyone out of your way. This doesn't work very well if everyone loses radio communication simultaneously.
Could you in an emergency, if you had no other options? Yes. But ATC can't legally give you a clearance that was dependent on using ADS-B for separation.
(Similarly, if there's a potential traffic conflict, and you tell ATC you see the other traffic on your ADS-B display, they just have to disregard that statement and assume that you can't see the other aircraft at all.)