Yes, I see what you are saying. I think having leads from the black/white wire spliced into the turn signal wires with a diode on each side (allowing current to the indicator bulb) is a fix. I don't want to remove the indicator - the bike is too old to trust the electrical system without an indicator. But,I think there may still be some indicator light glow as mentioned in the instructions when using diodes. This is a bit of a head scratcher and one of the reasons I wanted to understand a bit about what the relay does.
Another option is adding two relays (one for each side) that energize the indicator bulb when each side flashes. That may stop the indicator light glow described in the instructions by isolating the left and right flasher wiring. Other than finding room for the relays, they aren't expensive or difficult to wire.
Ok. I need more time to think of a more complex solution. Damn, I thought this might be simple. I thought the relay may operate on a change in the resistance, instead of current flow. But, the simpler approach of starting a timer when current flows is better. Simpler is always better.