It's a good question, but that is not the same issue as the one the article is about. The spiral arms maintain their shape not because of an anomaly in orbita

It's a good question, but that is not the same issue as the one the article is about. The spiral arms maintain their shape not because of an anomaly in orbital velocities, but because they are density waves[1]. Kepler doesn't work because the arms themselves and the rest of the disk exerts significant gravity. [1] https://en.wikipedia.org/wiki/Density_wave_theory https://en.wikipedia.org/wiki/Density_wave_theory