That may be true. Let's say that it is. In that case, the article could be improved by distinguishing between the two cases of an author with a niche brand of some sort and an author without. If you have a niche brand, some of the other marketplaces may offer advantages such as lower costs or more flexibility.

An author with an established brand or significant following (and I count your HN reputation as an example) can sell anywhere, b/c readers will seek out his work and deal with all kinds of annoyances: usb cable device syncs, finding out which folder to copy into, which ebook format file to use, etc.

If you don't have a brand or following, though, selling through marketplaces that are not tied to devices is a huge disadvantage.

The typical ipad/kindle/nook user does not go outside the store built-in to the device looking for content (the majority of them probably don't even know it's possible), except for a small, bleeding-edge minority.