A red flag is an indicator that something is wrong. It’s not a deal breaker, though red flags can also be those. These are different concepts with some overlap. A red flag in this case would be the developer stating they don’t care about non-windows systems.
I think that calling it a redflag is a little out there when you buy a mac. Who buys a mac expecting to game on it? That’s like complaining about games on steam not working on Linux in 2012.
A red flag is an indicator that something is wrong. It’s not a deal breaker, though red flags can also be those. These are different concepts with some overlap. A red flag in this case would be the developer stating they don’t care about non-windows systems.