That manual is very odd, it has unnecessary repetitions, spelling mistakes and seems not to be written by a human. But the basic approach seems to be not to unrealistic since I’ve found similar descriptions on other websites. So it seems to be a usual way for other phones.
On these other pages I read about commands like “fastboot OEM unlock” and the like. I tried them all which didn’t work so far.
However, if it were possible, then I gues I’m already failing at the point to enable “OEM unlock” in developer settings. This setting is simply not there even if search function claims to have found it. It is also mentioned in Asus forum by other people.