In and Yo are, at least here on AikiWeb, more and more seen as contradictory forces. I suspect that it is in effect the influence of Western thinking. Most texts that I know of speak of In and Yo as forces that join together (musubi) to create something new; a child, a new technique, a new art, Or experience something afresh, like a new day, a homecooked meal with fresh vegetables, the taste of this years first wine.
Of course they join together - the basic Yin/Yang theory of the unification of opposites goes way back across the ocean to China, but I'd note the what you're unifying are, in fact, opposites. I don't think that Marc is really trying to say anything different.
I will note, however, that the theory has them joining together - but not becoming the same thing.