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I suppose when you look at it, Royd was able to replicate Juha Bach's physical appearance, not abilities like his brother Loyd, but that might have also given him the physical traits of Bach which allowed him to defeat Kenpachi since Bach > Kenpachi.
I was discussing this on another forum. Imagine if Royd replicated Superman's appearance, he wouldn't have his abilities to melt things with his eyes but his body would probably physically allow him to lift buildings and other heavy objects.
Although I suppose there is a chance that Bach fought and beat Kenpachi, then Yamamoto confronted a transformed Royd. Must have made a bloody fast switch, though.