But on the other hand, if you wanted to divest from oil with the expectation that oil will become a non-essential resource, you wouldn't go around proclaiming it loudly. I had the same thought when I first heard the announcement; this is going to be interpreted as more than a diversification move, and I'm not certain that it _is_ purely a diversification move.
Saudi Arabia is doing the same thing by listing Saudi Aramco on the stock exchange. If oil will eventually become a (relatively) niche resource, the market value derived from non-extracted resources will tank, and divesting needs to happen now, before everyone realizes what's going on. To do this without triggering the very thing you are hedging against, you need to have plausible deniability, which Norway does.