30 years ago, that information had some meaning, ie, crossing Cornish with White Rock. After 30 years of genetic study, experimentation, selective breeding, harsh culling, and detailed science, the birds of today are far, far beyond anything you'd get with that simple cross. In fact, to achieve the "nick" of the final cross, the grandparent stock is highly selected and bred, making the CX effectively a 4 way cross. This is widely practiced in the super-hyper layer birds as well.
No way you can make a true CX or ISA Brown in your backyard. We'd love to, perhaps, but just cannot. We simply do not have access to the grandparent stock to create the parent stock to create the final bird. These genetics are patented and closely guarded secrets of the poultry genetic companies. These are billion dollar, world wide businesses. The writers who state that these super layers are merely sexlink crosses of an RIR over a RIW simply do not have all the facts or their statements are so over simplified as to be useless. Ditto with those who believe the CX is a somewhat basic cross.
Can we "create" some good meat birds? Yes. Can we also breed for and create some good layers? Certainly. But we are quite unlikely to duplicate the results of a million dollar poultry research facility.