That picture as is the one of the points should be self explanatory and testament to Spanner's and Scottly's post. My signal lights are wired the same as yours on my F1/2/3 also. And As pictured in the modern photo, they must rely on galvanic current (dc) flow through the bulb's base, socket, and bracket, all of which may be dissimilar metals. Which would still be exposed to your galvanic corrosion concerns regardless of the wire Honda provided. The point plate picture references the above posts indicating manufacturers have incorporated frame, engine, chassis, grounding circuits utilizing similar and dissimilar materials in their primary designs for some time and still do.
I'm basically agreeing with this here. Galvanic current is basically any DC current
may not be not related to Galvanic corrosion.
Added in the clarification as galvanic corrosion likely does have a very small current flowing as part of the reaction.