I think myths like this originate from people who have tried to jump start vehicles improperly and caused damage to their electrical systems. I've seen many people hook up jumper cables in very dangerous ways, including backwards and in the wrong order.

Using a jump pack limits that danger as it removes the 2nd vehicle completely and simplifies the process.

With jump pack off and vehicle ignition off, hook red to red, black to black. Power on the jump pack, wait until it signals it's ready to provide the boost, then start the vehicle. Turn OFF the jump pack, then disconnect black, then red clamps, although with the jump pack turned off, the last step isn't as critical.

When using booster cables:
Red to dead battery first, then red to support vehicle.
Black to support vehicle first, then black to dead battery.
Then start the support vehicle and then start the dead vehicle.
Remove in reverse steps.

I see a lot of people hook up both cables to the support vehicle while it's running, then hook up the cables to the dead battery, causing sparks, which can damage the electrical circuits of the dead vehicle.