Note also that the quotes statement from the Magician's Way can also be interpreted as a restatement, not a new allowance. As in, "remember, Mystic Adepts can take the magician metamagics too." I'm not looking at the book right now, but if I recall correctly, that part doesn't include wording that indicates it explicitly as "the Magician's Way also gives this ability to Mystic Adepts."
Plus as others have pointed out, the books never specifically indicate that an adept cannot learn metamagics through the arts just like magicians.