Like others have pointed out, the nomenclature is a bit off.
Horford is primarily playing high-paint defense for Stevens. He has primary responsibility for calling out the p&r defense and for switching (or not) and for showing / doubling on the perimeter. Stevens gives him this responsibility because he is the best of all our bigs at those things.
I'm not impressed by Coach Nick's 'analysis' here. He keys off the Net Rating difference between the two front-court pairings, but ignores why the sample sizes are so dramatically different. It is a naive mistake to think that if you simply swapped Olynyk for Amir that the starters would suddenly be posting a +22 Net Rating.
I do think that, as Olynyk's game continues to improve as he gets further from his shoulder surgery, that we may get to a point where it makes sense to start him over Amir, but I don't think we are all that close to that point yet.