How about this for a possible explanation:

While the business is profitable, most of the profit has been from a few large whales. Whale sightings have been getting rarer and rarer. DN worries that BTC whales are going extinct, which would leave him with very little volume, and so he figures it's better to sell 30% of the business for 2000 BTC now while he can boast of relatively high profits over the last year.

That would seem to make sense. While he could end up missing out on a bunch of value if betting volume doesn't die off, it's a hedge against that eventuality.

Much the same as when the commission model switched from taking a percentage of investor profits to taking the commission up front, as a percentage of the *expected* profits. That was a hedge against the site performing badly. It turned out to be a bad decision, since the site performed way better than expected, but it did limit the site's worst-case commission take.

tl;dr: selling 30% of the business now makes sense as a hedge against possible future outcomes