https://www.youtube.com/watch?v=L_a8_zxYLik
They've since sold about 20M copies at $20 (now $30) apiece. I started playing in 2017 with Factorio 0.14, when the graphics were better but still decidedly retro and they'd sold maybe 4M copies.
The difference is that Factorio has a unique gameplay concept that is both extremely addictive and not really found in any other games. People will overlook shitty graphics if the game has a compelling concept that they can't get elsewhere.