Can you give us more info....like the name of the place so that we can research this? I would think that DNA testing of each and every plant would be quite costly--it is not like just ascertaining the cannabinoid levels and really what does this tell you? Seeds can and do pass on different chromosomes, so the only valid testing could be with the clones. A batch of seeds is always going to have some variations, just as children do.
The bottom line is that I really don't understand what the testing will tell you....that you have a clone that is x percentage of this and y percentage of that? So what....what good does that do you? Rather than getting this twisted up about picking exactly the right pheno, get your space in order pick something with characteristics that you like, check that the seedbank/breeder is reputable and find your own special phenol. Also, keep in mind that fem seeds are not 100%--they can and do turn out to be males sometimes or they hermie, which is worse.