So as an uneducated layman can I assume that the sockeye are infecting the farm fish?
That is how it happens considering adult Sockeye are natural carriers of IHNv, having immunity at that life stage; whereas, Atlantic Salmon have not developed the level of immunity and are therefore very vulnerable to IHNv (the virus) and IHN (the disease). When IHNv is confidently suspected on BC fish farms they be proactive and will cull their fish, before CFIA confirmation, to avoid risking it spreading to other farms. This is one of the reasons why IHNv from farmed fish to wild Sockeye is considered minimal risk.