They did nice work. It is similar to what Vance Whitaker and Stan Hokanson did a few years back as well. Vance and Stan focused on combining ability and reporting their results as "general combining ability" explaining more than "specific combining ability" for black spot resistance. In other words, it seemed predictable that parents with high horizontal resistance tended to produce seedlings with average higher horizontal resistance. If "specific combining ability" was found to be the case, then it would be more unpredictable to find high horizontal resistance in seedlings and a person would just need to try more combos to find the specific combos that would provide high levels of it.
The study in Texas found predictability as well and found moderate heritability, which is great. Heritability is another way to showcase more specifically the kind and extent genetics serves in breeding for a trait.
Horizontal resistance (when infected with black spot roses can slow the growth of the fungus) is a good thing to aim for as it seems much more durable than race specific resistance. Of course, the best is to combine both.