2) Taking his argument in good faith, he also means "free" apps with microtransactions, because in both cases the same situation exists: The iOS App Store is too big a market to ignore. Which leads to the same problem, regardless of whether your app is paid or "free", that if Apple suddenly decides it doesn't like you, you're screwed.
What I've noticed on macOS however, is that is a lot easier to sell things that should be bundled or FLOSS for relatively serious money. For example there are dozens of rather expensive "Finder replacements" on macOS and you'll have a hard time selling something proprietary like that to Linux users because we do have good file managers that are libre software already.
- sound editing/musical production
- video editing/special effects
- CAD/CAM/EDA
- Mechanical/physics/electrical simulations
- interfacing with custom hardware
Hollywood studios run on Linux (except Pixar, and granted, not with webapps), so it is possible. What's missing is the middle-ground, mass-market solution like Premiere/After Effects or FCP, though DaVinci Resolve is quite nice entry there.
- interfacing with custom hardware
From experience, that's a lot easier on Linux.
As for CAD, video editing and music production - sure you do have a point, except that in those areas the likes of Autodesk and Adobe dominate anyway and the "pro" segment of macOS/Windows users is a tiny fraction of the overall userbase, so I'd be rather surprised if that's the kind of software you can sell in decent numbers to macOS/Windows users as an indie dev.