Matt Henry is bowling brilliantly these days. And his splendid form has helped New Zealand to make up for the vacuum that was created as a result of the absence of more experienced pacers such as Trent Boult, Tim Southee, Kyle Jamieson, Adam Milne and Lockie Ferguson. BTW, I am still surprised how this small country with a population of less than 5 million can produce so many quality pace bowlers. And this is not a new phenomenon. Remember Richard Hadlee? Danny Morrison was another great bowler.
I don't consider 5 million to be a small number and out of that population even if there were hundred quality pace bowlers over few decades, it's no big deal. Also, it's easier for smaller countries for to see the talent and importantly nurture it than bigger ones.