re: Isn't that a double negative?
Anyway as the people who have offered up this theory actual did the math required to have an expert opinion I think I disagree with you. Unless you have some numbers to share.
I imagine it's possible to have sufficiently large object that some part remains and as it blasts a crater and blows vast cunks into orbit. Similar to the way many believe our Moon formed.
I think your problem with this idea is that some part of the two moons of Mars would logically also be made of Martian materials and their density is to low to be rock alone. If fact many hypnotize it would have to be mostly water or some similar density elements.