Actually it is not true. I'd checked whether it is available for me on my iPhone and there was the "Turn on Two-Factor Authentication" button. However, when I clicked on it, it said something like it is not possible to activate it at this time. Few days later I went to appleid.apple.com (wanted to change my password to make it stronger) and I saw that I can set-up two step verification (I didn't know at that time it's something different). I activated that (works well) and now I would like to know how to try the Two-Factor Authentication (it doesn't show up on my iPhone/iPad anymore). I've also checked my wife's iPhone (different apple id) and there is "Turn on Two-Factor Authentication". However, also doesn't work.
So the think is that the link, you've provided (I've already saw that before), doesn't show the correct information. If the iPhone shows the button, it doesn't mean it is available for you. But if you set-up Two-step verification, there is no way to check whether you are able to set up Two-factor verification or not.
It's so confusing...