Glorantha was made from chaos, but it was made to be different from it.
Chaos doesn't mean to change glorantha, but turn it back into chaos, by breaking down distinctions between the various things in it(runes) and turning it into indistinct stuff, and then turning that into nothingness. Chaos isn't change, it's destruction.
Umath didn't destroy anything, he made somthing new(air), chaos doesn't make new things as much as it destroys barriers between things. Undead creatures are the result of the difference between life and death being destroyed, not really something new.