Diacetyl is most commonly caused by alpha-acetolactate, which most beer yeasts produce, but then reabsorb near the end of fermentation. If the alpha-acetolactate is not "cleaned up" after fermentation diacetyl can result. Some strains produce less alpha-acetolactate than others.Some have even been engineered or selected to create almost none.
"Green beer" iscaused by Acetaldehyde which is also generated more or less by most brewing yeasts. Some strains produce less than others. Acetaldehyde usually fades over time, the less that is produced during fermentation, the less of a problem and the shorter the aging time to remove it.
My guess is that kviek doesn't generate very much alpha-acetolactate or acetaldehyde, or it is very good at cleaning it up.