You are missing the point. What makes a super a super is not necessarily how it looks. Its when having two genes is expressed differently than having one. If a het and homozygous looked the same but the homozyguous could breath fire then it would be a super and therefor the trait would be co-dominant. The point is that dieing is what makes them different and therefore its a co-dominant morph.
Homozygous spiders would make up 25% of all spider to spider crosses. The fact that we don't see them means that its most likely lethal. You can't just say they don't exist. The genetics makes them relatively easy to make. The fact that they are not here is the proof.
Thats like saying that the Amazon women of legend didn't have male children.