Incorrect. The Patreon is people who donate money to the game. Using THAT as a sample would be a selection bias, as those are clearly people who like it enough to keep buying it.
No, a free game with optional earlier access to newer content via patreon and steam means that
both steam and patreon are donations to the game.
It isn't really "buying" if the dev lets you download it for free.
Also, selection bias is any case where your sample is non indicative of the population.
What you are looking here is plus ultra extreme deluxe selection bias on steroids.
Regular selection bias:
"I interviewed random people 5 minutes away from a major university. which meant I was % more likely to encounter asians compared to population at large"
This case of plus ultra selection bias:
"After giving people the product for free, I asked who amongst them is willing to pay 10$ to leave a review"