Knocking on wood was about knocking on unpainted wood, so that you have actual direct physical contact with wood. Remote knocking seems to go in the other direction...
There are societies where rather than knocking on wood one turns one's head and spits. For example: "Grandad is looking well after his fall, let's hope that doesn't happen again. Ptew!"
The knockers are missing solar panels to continuously charge their batteries. So there will be sustained costs to keep them running. And how water-proof are they? Apart from that, it's going to sell like hot cakes.
Looking for this in app form.