And then when Apple adds it to the existing OS functionality demoed at the beginning of the video, we can have a different embodiment of "the problem of macOS", and look forward to articles about how Apple is "sherlocking" small developers again.
More seriously I don't understand what the problem is here. Is the problem That some functionality or another wasn't built into the OS? Most things "linux" does aren't built into "linux" or even "gnu/linux", they're a distribution composed of both some combination of linux kernel, base tooling apps and then improved UX apps on top of that, all of which will vary greatly in what functionality they provide out of the box. What makes this particular app and functionality a critical piece that an OS should provide?
Is the problem that the developer is charging money for their work? Why is that a problem? For the life of me I don't understand why software developers seem so hellbent on devaluing their own work. Why is it outrageous to spend $30 for something like this if the built in window management functionality doesn't suit you? If you're broke, or live somewhere where $30 USD is a lot of money, sure you might be annoyed that you either need to code this up yourself, or dig for someone offering a free replacement, but if you're not either of those, $30 is a hair over dinner for 2 at a fast food restaurant these days. It's (likely less than) a tank of gas. It is 1 hour of work at the median US developer salary of $79k / year. If you could code something like this up in less than an hour, maybe it's not worth your time, but if you can't, it's a screaming deal if this is something you need.
Software is worth something, human labor goes into it, and those humans need to eat and pay their bills. Maybe big corporations with deep pockets and huge recurring revenue streams have warped our views a bit with the amount of software they can give away for free. Certainly the amount of benefit we have all received from countless people volunteering their time away at various open source project has spoiled us some. But those people can only volunteer their time because somewhere, someone else is paying them for other parts of their time. If we the developers constantly treat our labor as if it has no value, why would we expect anyone else to treat it otherwise?