The FTC site doesn't seem to be a law site, but tries to explain things to lay people. And they are a bit careless at that. Have a look around on this site, and you often find questions "if I do X, is that legal or not", and every answer starts with a big "it depends". That's missing on the FTC site, which makes their answer quite misleading.
One way of doing business that has gone out of fashion was this: You as a business intentionally send merchandise to people who never asked for it. Then you ask for payment, usually a lot more than the merchandise is worth. Some people don't know what to do, don't know how to return the items to you, and when you send a bill and threaten with lawyers, they don't know what to do and pay. That's what the FTC site covers: In this situation, you have the right to just keep the merchandise without paying. Knowledge about this has been going around a bit, so you can see why this has gone out of fashion.
But the other situation is that a business is selling stuff legitimately. Someone (not you) ordered merchandise and paid for it, and due to some mistake the merchandise is sent to the wrong address. That's a totally different situation, and you have absolutely no right to keep merchandise that was destined for someone else. That's what your second quote covers: Keeping that merchandise is plain theft.
Now look at the FTC site again. What they say absolutely covers the second scenario, but they are not a legal site, they are just plain careless. Dangerously careless. What it should say: "If you receive merchandise which you didn't order, and which is intentionally sent to you in order to make you buy it when you had no intention to buy it, then you have a legal right to keep it".