No, it's most likely not a coincidence. Viber intends to show the person as "unreachable" not as "disconnected from the Internet". Basically, Viber's calls and video calls use the same computer resource as the calls made via Facebook or other similar services. This means that when you're in a Facebook video call, the resource is no longer for Viber, which notifies others that you can't be currently reached by displaying you offline.
