No steel plates. You can just put two more wood members one on each side. Based on the loading required in the top chord will tell you how long the members have to be. You essentially have to get enough nails, lag screws, bolts, Simpson wood connectors, or fastenmaster connectors to transfer the load from the old member to the new ones and then back into the old member on the other side of the crack. Technically the chord has limited tension capacity in bending if it has a crack on the bottom which just so happens to be at a knot. He's not entirely wrong about the capacity being degraded but a whole replacement truss seems a little far fetched. Also technically if it were a true truss only loaded at panel points the the top chord would only be compressed so it may not affect strength at all. However we rarely ever see true trusses used in real construction except maybe for long long span trusses.
Hope that helps