....not technically true
Angels are manifestations of white-aligned ideals, formed of white mana, however it's not only possible but somewhat common for angels to be able to have red, green, or blue mana within them as well.
For instance, righteousness is a white-aligned ideal, and therefore an angel can be a manifestation of righteousness. Red, however, also has its toes in that pool, so in the right circumstances said angel can be a R/W angel, as seen in Ravnica.
W/B angels are somewhat of an enigma, as it's unclear as to whether they are capable of forming in a similar manner to W/R, W/U, and W/G angels, or if they're existence is relegated purely to the corruption of their being. We know for a fact the latter is possible, but the former is unknown.
Demons vary from plane to plane, but they are more corporeal than Angels and while their origins are similar, rarely are they held by the same restrictions as angels. Demons are much closer to other beings in how they operate than angels.
And technically, while that's true in our reality, as seen in Planar Chaos there exist parallel multiverses in which male angels exist. And I believe Doug has said in the past that a male angel isn't impossible lorewise, but for stylistic reasons they don't really want to.
But yes, "Angel Demon" makes no sense.