I don't know where he lives, but in California it could easily add up to close to 50% since he's "rich".
Still, yeah, having the entire amount being dropped on them in a single year will probably have pretty major federal income tax consequences. Hopefully, they will figure it out, and the sales of Dwarf Fortress on steam won't dwindle. I find that it is a pretty good timing for the release, given that Rimworld (the spiritual competitor) has gone extremely mainstream, so people are definitely more open these days to that type of games.
The financial assets will return money in the future, but much more slowly, and get preferential tax treatment compared to direct income (such as long term capital gains tax discounts).
When you have $7mil in cash (and expectation of more in the future), there would be options not available to lower net worth people i suppose?