He isn't a millionaire. I did what you just did and copied and pasted the stats you copied and pasted. You d as f.
This stats is the same stats as from when he started to sell the game on steam.
Also these data are not the true data. As an example.
Quick stats as of today
230
active players (38 min ago)
258
active players (24h peak)
96.7%
positive reviews
$3,1m
gross revenue
304k
units sold
48.8 hours
avg play time
28 hours
median play time
the game is 4 years old. He is making less than 1million per year, Lets say he makes less than 100k per month and he has costs every month towards creating this game. You don't really know how much he earns. With that being said, he isn't even a millionaire yet.
Also this data you took from some webpage called Video Game insights. There are others that says his gross revenue untill now is 2.6million. Everyone will estimate a value, that doesn't make any of these datas correct or reliable.
This doesn't make him a millionaire or multimillionaire as you said. "Oh, I know DPC, He makes games only because he likes it, he enjoy. Not because he actually need the money". And untill few months back he was relying on money of people on patreon to buy a new pc. Such a millionaire lol.