They were also making 14-15K per month before that (it's mentioned in the post), so they were already extraordinarily successful as far as 2-person indie gamedev teams go.
But that also means that as long as they don't significantly increase the cost of their lifestyle, that $7m pre-tax will go a very long way.
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?
People are fickle and hindsight isnt always so clear.
There is also a Nintendo Switch release, so they will have to pay for the Dev-Kit's and licenses which are not cheap. Not as expensive as in the 80's, 90's, early 2000's where a single Dev-Kit could be six figures, but still a significant expense for a indie studio.
So we are already a little bit off from the numbers you posted. I assume, the 30% you cut off is for taxes. Good idea, but Steam and all other platforms where the game is sold also want a piece of the cake. I don't know the numbers for other platforms, but Steam takes 30% of the listed price.
So even with a $30 price tag on Steam, they get $21 after commission. An that is pre-tax... They need to buy hardware for their developers, rent an office, pay developers and so on. So even the founders are not getting rich. A "boring" job in business software can make you more.