Fixing a mistake takes several times longer than typing the letter correctly: the mistake has to be noticed, the hand has to travel to Backspace and then the place has to be found again. A typist who makes three mistakes in a line is slower than one who types calmly and cleanly.
This drill will not let you continue with a wrong letter. That is deliberate: typing is a habit of movements, and a wrong movement that goes through settles in exactly like a right one.
If one particular spot keeps going wrong, slow down there. The rest of the line can come at full speed.
The drill uses long words, because that is where sloppiness shows. A short word can come out right by luck; a long one cannot.