Protos
Die Schwarzbier Polizei
I think the secret ingredients in that Rongoteus beer ("a Finnish Rye Dubbel") were lower IBUs, as you noticed, and also a healthy dose of Rye supplemented by Black Malt (Black Rye Malt in my case). The beer came out so chewy and strong-flavoured that the Voss twang was well-balanced. Also, at 1.070 OG and 7% ABV it was in the traditional strength range for Nordic Farmhouse Ale, which well compensates any kveiky tanginess. Yes, it didn't change its flavour with time (it seems Voss doesn't age into more tartness, unlike Hornindal).I'd like to add that the one stronger traditional Rye kveik that I brewed after your recommendation, which was the best Kveik for me so far, totally nice beer, was still very good even after a few months in the bottle.
An excellently designed beer recipe by Mika Laitinen, I'll rebrew it some day too.