Trans doesn't have to be binary in the way that you are a trans woman or trans man in the traditional sense of a cis gender binary (i.e. that there are women and there are men and that is that).
If we say gender is a spectrum, it means there are cis men and women, there are trans men and women, and there are people who don't quite fit either of those positions. That seems to be where Monroe is coming from, that they are trans but they are not part of the traditional cis/trans binary that we've been shown in the media so far. Agender people fall somewhere along this spectrum, as does anyone who calls themselves gender fluid, genderqueer, people who are intersex, and so on.
I don't much like the idea of a spectrum for this particular subject, tbh, because it suggests cis is at one end and trans is at the other in some kind of 'opposite' position to one another. I don't think that's the case. I prefer thinking about it as a ball, I suppose: imagine a ball and you can draw a dot on it with a felt tip pen, and that is where you locate your gender. Someone else can draw a dot somewhere else on that ball and that is where they locate their gender. This ball can fill up with dots, and none of them have a more preferential position in relation to any of the others, they all just exist on this ball together, just not all in the same location. The metaphor isn't perfect, but it helps me to conceptualise gender locations in that way rather than a spectrum.