I doubt the member you know that earned $1200 per month was a sole case by being on a customised deal. If I recall correctly, for a short period at one stage all the Chipmixer participants were getting paid more than $1200 per month because the price of Bitcoin was high enough to take them over $300 per week.
I know someone who earned $1200 per month from ChipMixer.
We are going meta at this point, but it's a little incorrect - It was max 0.0375
BTC per week until some time in 2021, and then up to $300 per week after that.
So in any case, it was quite a number of people, not just one.