XGE has stated that perceiving a spell is dependent on the spell having a component. It further states that a feature removing the component requirements makes it imperceptible. Subtle Spell doesn’t remove material component, so a spell with “M” doesn’t benefit?That's correct.
— Jeremy Crawford (@JeremyECrawford) November 18, 2017
HOW?? How do they know? Of all the answers this is the most ambigious one. How????