I have always thought that the main definition of "species" is that cross breeding led to infertile offspring ( as when horses x donkeys = mules) or no pregnancy at all ( most species crosses) , or that behaviour differences didn't allow a significant amount of cross breeding - like mule deer and whitetails.
So giraffes have been geographically separated over time, and have developed a few genotypes, with four distinct phenotypes. That seems like the definition of subspecies to me.
Many scientists don't think that European Red deer or Asian Maral stags or North American Wapiti ( our Elk) are different species, just different subspecies of Red Deer.
So he's entitled to his opinion, but others with different opinions may be correct too, it just depends on the " school of thought" they belong to.
Splitters vs Lumpers
Not too many lumpers and splitters to be found these days. The fact that they detected no evidence of inter-breeding suggests one or more isolating mechanisms at work. Molecular genetics have pretty much become the deciding factor in the definition of species.
