That guide is written for a country where the AC is at 220-240 volts, so the information about the power switch isn't always right for North America, where 120VAC is much more common. 240VAC switches disconnect both AC lines because both are "hot" (at high voltage), but in the US the switch in almost all PSUs disconnects only the hot AC wire (black, 120VAC) but leaves the neutral AC wire (white, ideally 0 volts) connected. Normally there's 120VAC on the black and close to zero volts on the white, but that's not always true, especially when the building wiring has been installed backward. So it's best to leave the AC power cord unplugged from the wall outlet while working on the computer.
It's also false that it's better to have the computer case grounded to the wall outlet while working on it because what really matters is the voltage between you and the computer stuff, not the voltage between you and earth ground. You want to be grounded to the chassis, not to earth. Also notice that anti-static wrist straps have at least one megaohm of resistance between you and their ground connection. That's a safety measure, in case one side of you touches high voltage while the other side of you is connected to ground through the wrist strap. Because of this some electronics factories don't let workers to go through an entry checkpoint unless they plug in their anti-static wrist strap into a tester and the tester shows that there's about 1 million to 2 million ohms in series.
A few power supplies do not have their case ground tied to their power ground (black wires that go to the motherboard, drives, video card, etc.) because that's actually supposed to be the proper thing to do. In other words, the chassis ground and power & signal grounds are different. Original Antec TruePower and SmartPower models had an option for this -- a bare wire jumper was soldered to a circuit trace for one of the mounting screws of the circuit board to connect the chassis and power grounds together.
Get your information about electrical safety from electronics people, not computer people, who are usually not electronics people. For information about anti-static precautions, google "ESD" (ElectroStatic Discharge), or check the last chapter of any chip databook from National, Texas Instruments, Linear, etc.
I don't use an anti-static wrist strap but instead cover the whole table top with anti-static bubble wrap or anti-static foam wrap, allowing it to drop down a few inches in front of me so I'll be more likely to discharge myself through it. Set everything on that surface, and work in short sleeves so your elbows will likely discharge yourself frequently. Staples and Home Depot sell anti-static wrap. Do NOT substitute aluminum foil or other metal because they can easily short out the lithium battery on the motherboard and make it rupture.