I used Strava to time a 1.5 mile technical climb, using the same power on both bikes I went approx 4 minutes faster on my StumpJumper. Granted this was different days, but every time I've tested it the Hei Hei is slower.
Same power? Using power meters on both bikes? If you're not using power meters, then you're not measuring "power" and it's irrelevant.
4 minutes difference in 1.5mi? That's kinda huge. I have two bikes that are WAY more different than the two you're comparing and when I take each one out on a particular section of similar length, the time difference is seconds.
I think you're conflating some things and failing to consider others. Folks have already mentioned setup.
Also since you're talking about technical climbs, it begs for lots of details about the section of trail you're considering. What is "technical" in this discussion? It's going to matter.
The two bikes are very different beasts. The Kona is basically an xc bike, whereas the Stumpy is a more rowdy trail bike. The extra suspension of the Stumpy will let you plow through stuff without much finesse, but the shorter travel on the Kona requires quite a bit of finesse in technical trails. And by finesse, I'm specifically referring to rider input to maneuver the bike.
We have no idea about your riding skill, and even less idea about your riding style.
I can make a few guesses based on what's been said so far (and what hasn't) but generally speaking, I think the difference is the rider.