Illusions dont cut, Asura's extra swords do.
So either we gotta redefine what illusions are or we gotta classify Asura as non-illusion.
nothing needs to be redefined, you simply don't know what an illusion is .
illusion: a thing that is or is likely to be wrongly perceived or interpreted by the senses.
zoro doesn't actually grow two heads and spare arms , he doesn't manufacture two new wados and enmas either.
by definition that's an illusion because they are seeing things that aren't actually there .
so why do the extra swords that people see feel like they cut?
safe to assume his haki in ashura is much greater than his regular attacks so it infact does feel like he is using 9 swords rather than the 3 he has always held on to.