For AAA titles I could understand that, but for indies?
They won't sell that much Windows copies, so having a Linux option is always good (see other indies, like Factorio, Oxygen not included, Hollow Knight).
It's especially true for indies: if you don't sell as much Windows copies, you won't sell as much Linux copies too, proportionally. I've worked in an indie publisher, and not a single one of our projects would return the investment of releasing a Linux version.