Extention cords are dangerous because they are not always the right gauge.

People get cheap extention cords and try to pull more power than the cord can handle and it ends up heating the cord and causing a fire.

It’s why a lot of things say “Do not use an extention cord! Plug directly into wall.” Because they know someone is going to use a dollar store extention cord on a space heater.

Not because the space heater won’t work with an extention cord

Most people care about plug type (low profile) or color more than they do the quality or gauge of the wire.

Anyways, this cord could be dangerous like that if the gauge isn’t the same or thicker than what’s in the wall.