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.
You recall correctly! But it wasn't because the Bitcoin price was high enough, it was guaranteed over $300 per week if someone made the allowed 50 posts to be paid as it was a flat rate for every participant of $6 per post. Depending on the month, it was ~ $1300-1350 per week. In many places of the world, an account in that signature campaign would get them above average income per month.