Yes, he is correct and he is referring to the absolute values of number 3 and -3. And by absolute value, this is the distance of the number from the origin zero (0) which is symbolized by two vertical lines, as |3| or |-3| is equal to 3.
the picture shows a number line where green is the origin zero (0). The purple line is the distance between 0 and 3 which is 3. The pink one is the distance of -3 from 0 which is also 3. Therefore, |3| equals |-3|