I have made at least three different trips to Dallas over the last few years (and some of those trips two days at Dallas Zoo) and not once I have seen the okapi anywhere other than against the far back fence. You are either lucky or you know how to make the ultra-low frequency okapi call to attract them.
No, but unless he has a ten grand 600mm lens with 2x teleconverter, there is no way this shot would have been possible from any of the places I have seen them when I was there.