Finding the current of an open circuit for Thevenin Equivalence
Saw the link you posted. They are never saying that 1A flows through the resistor 20 ohms. The current is 0.2 A when open circuited. They just transformed the voltage source to current source to reach the thevenin's eq. circuit. 

This is how a voltage source is transformed to current source.



In place of R, we have 20 ohms and in place of Vs we have 20V source. Everything to the right side of A and B would be "external circuit" while transformation.