What happens when you tighten the bolt is that you clamp the collar (#12) to the frame, nothing should ever pivot on the bolt. The swingarm should pivot on the collar, the bushings (#9) are its bearings.
What mark has explained is that with a lack of greasing (and a rather poor design, really) the bushings, tight in the swingarm ends, rust/gall onto the collar. That gives a very stiff swingarm, either it pivots poorly on the bushings or it manages to pivot on the bolt.
Normal wear makes the collar oval as well Too oveal and there will be ;ateral play on the wheel and handling will get weird.
Several things can go wrong: that rusting/galling, wear on the bushings, and wear on the collar.
The collar is unavailable new.
Phenolic bushings are, I think, available from Honda but I don't think the steel ones are.
Mark reconditions the assembly by making custom fit bronze (oilite) bushings - maybe he regrinds the collars to be round, as well.
You can get aftermarket bushings but, like Mark says, they tend to be a very poor fit. Usually too loose, the makers are probably aware of the fit issues (the inside bore is reduced when they get pressed into the arm ends) and make them so the swingarm and frame will go together... but to avoid a tight to impossible collar fit they make the ID oversize ... there will be excess lateral play at the wheel.
A decent machinist could make a replacement collar but it would be pricey!