Actually the Cornish X is not a cross between a Cornish and White Rock. They were developed about 70 years ago and Cornish and White Rock were two of the parent breeds (may have been others also). But geneticists have improved them over the years to where they have the traits for very fast growth we see today. It's developed to the point that it takes 7 different flock to produce the eggs that hatch into the Cornish X birds we eat. Four flocks produce a specific grandparent of the Cornish X, their offspring are combined in a specific way to produce the parents of the Cornish X, then the one flock that lays the eggs the Cornish X hatch from. This article talks about how it works in Australia but the general principles are the same. So yes, really expensive to produce them.
https://www.chicken.org.au/chicken-meat-production/
Most of the research is not in genetics, they have that pretty well in hand. The research is primarily how to be most efficient in keeping the Cornish X alive and rapidly gaining weight and how to manage the parent and grandparent flocks to keep them alive an producing fertile eggs.
The Cornish X are hybrids and will not breed true, that part is correct. You will not get pure Cornish X if you breed males and females. But as Ralphie mentioned, they do have the genetics for very rapid growth. They are still going to grow so fast your challenge is keeping them alive, though some will grow a little faster than others.
I looked at those Kabirs, they look a lot like the Rangers. Technically they are not a breed. A breed is recognized by your national poultry association, each country is different though some countries adopt the Standards of other countries. The Standard Of Perfection (SOP) defines what makes it a breed. Feather color is not one of them. Things like comb type, leg color, egg shell color, eye color, size, and especially body conformation is what makes a breed. Feather color is a variation. As an example, an Auracana in the UK looks a lot different than an Auracana in the US and each country has different approved feather colors and patterns.
Instead I'd call the Kabir a type, like a Ranger is a type. They have certain characteristics but a lot of leeway is allowed in them.