Not only is this assumption incorrect, the existent of transgressive identities creates quite the opposite effect. Masculine females and transguys create a needed incongruency in the cultural assumption that male=masculinity=power. When those born and socialized female identify as male, it destabilizes the biologically tied gender binary. FTMs have the opportunity to "infiltrate" and as men, call other men on their sexist assumptions. It also creates an entire subculture of individuals who have personally experienced both sides (and everything in between). Therefore, they are experts on gender and sexism, and have a wealth of insight and experience to teach.
Identifying as man or as woman does not directly translate to proliferating the oppressive binary. How is an FTM is any less revolutionary than a femme identified woman? They both identify with one end of the spectrum or the other but that doesn't make them part of the binary system. Any female socialized individual who can feel empowered by his or her gender identity, whatever it may be, is revolutionary in our patriarchal society. And just because masculinity has been constructed to equal power, does not mean that those who identify as masculine necessarily identify or even want that power.
feminism argue "we should expand the definition of female instead of abandoning it." But maybe instead of expanding the category, we should destroy it altogether. Instead of saying "masculine women are women too" we need to open up categories into a spectrum of identities where power is not tied to "biology" or gender identities. And until we stop dividing people into men and women, pink and blue, dolls and trucks, and start offering a spectrum of identities, sexism will triumph.