I don’t know for sure but 3.5M lifetime sales times 30 is $105M*70% is $73.5M, they have around 30 employees, maybe they pay $200K a year, so that’s $36M over 6 years, netting them $37.5M. I could be way off though.
The game wasn't always at $30. I saw it for $20 a few years ago (regular price) and I believe it was even lower before, like $15, but I'm not sure.
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.
