Um, isn't that a reasonable assumption if we assume orders denote genetic similarity?
It depends on your model of taxonomy. If you group based on inferred phylogenies, then yes, "order" refers to one or another degree of genetic similarity.
If you group based on morphology, though, then order doesn't necessarily mean that. It'll tend to, because phenotype stems from genotype, but it won't be guaranteed. Of course, morphology in all but a few cases is all we have to go on for fossils.