As I said in my post, IF you can achieve your optimal triathlon position on a road bike with aero bars, then you’ll be nearly as fast as riding that same position on a tri bike. But it’s a big “if” - tri bike geometry is designed to put you in a good tri position, road bikes are designed to put you in a good road position, which is different. Even if it can be achieved there will be compromises in other areas.
So yes, if you don’t have budget or space for a tri bike, and/or just want to finish your IM and aren’t too fussed by time, then you can certainly go fast on a road bike. But if you’re targeting a sub 10 finish and can afford another bike, then on a pretty flat course like Taupo a triathlon bike is 100% the best choice.