"Sine waves can not be stored digitally and still faithfully resemble a clean waveform."
The text mentioned (“Sine waves stored in ROM are used for the control waveforms at constant speed, and this achieves smoother and more stable rotation”) does not talk about clean waveform, but about smoother and more stable rotation. Both being relatives, "better when compared to..." in some way. It is definitely a marketing description, but does not promise eternal accuracy.