As for advertising on Reddit, I view it as a fool's errand on the part of advertisers given the extensive popularity of PiHoles and adblocking software among the userbase.
That said, my concern is that it creates an environment for misaligned incentives. If say, hostile foreign governments have their troll farms buying awards to increase visibility of divisive and inflammatory posts in key threads, that furthers their agenda and creates a revenue stream for Reddit that is fueled by creating an environment where that flourishes.
I'm NOT saying Reddit is actively trying to do this as I think they'd be horrified to find this happening, but they have a fine line to walk here from a product and business standpoint.
Re: your other point, I do digital advertising for a living. I can assure you that the vast majority of users on Reddit are not using PiHoles. While many may be using ad blockers, those largely don't work for native ads in the mobile app.