In this example, Greg says that it passes through the origin which I understand. But when passing through the origin, when x=0, shouldn’t the equation become undefined? But I also get that the line cannot pass through anywhere else but the Second and the Fourth quadrant.

I haven’t watched Greg’s videoes, but according to my math knowledge and skill level (Q 170), surely the equation does not pass the origin.

Just like you said, the origin is undefined for the equation.