I mean subsidize in the sense that it's an artificial transfer from the public to the private, in the form of an artificially-created monopoly.

Copyright is a subsidy in the way a trade barrier is a subsidy. There's nothing natural about granting a monopoly on stories but not recipes, or on melodies but not the shape of a jacket or a building. It's just a hand-out that certain industries fought for, successfully made the status quo, and expanded over time.

And it goes far beyond paying people for creative work. Mathematicians are paid for their creative work and so are fashion designers. But neither have the right to say what others who weren't party to any contract with them can do with their creative work. If you want to share a mathematical formula or a fashion design or use it in new work you're free to, and the world benefits immensely from that. Books, films and songs are much more encumbered.

When I say the tax affects long-form work more than short-form, that's just based on the observation that books and films are typically locked away behind paywalls and off the open Internet, while the kind of ephemera the article talks about is free for everyone forever.

If books were free for everyone forever after 2 or 5 years, more people would read books, and it would be easier to build habit-driven products out of books. Lower the tax, increase the consumption.

How is it a transfer from the public to the private if I privately create something and want to privately sell it?

Copyright allows me to monetise my private creation. If it did not exist, my first customer could distribute it to everyone else without penalty and I would make no money.

That is why it is an incentive, not a subsidy. Without it, I will not invest the effort to create it. No public property is used to subsidize my creation.

In the absence of something like copyright, subsidy would be required in order for much creative activity to occur. You have it backwards!