This statement is true because reliability and validity are two very different things. It is true that an assessment cannot be valid unless it is reliable. However, it can be reliable without being valid.
Reliability is a measure of whether an assessment will yield the same results at different times. In other words, let us imagine that you have a test that is supposed to measure a student’s reading ability. If a student takes the test one day and scores 75 out of 100, but takes the test the next day and scores 99 out of 100, the test is probably not reliable. It is not reliable because it gives such different values for the same student without enough time for the student to have learned.
Validity is a measure of whether the assessment tests what it is supposed to. For example, if I test a student’s reading ability by asking them to recite the alphabet, my test is not really testing what it is supposed to.
So, how can a test be reliable but not valid? Let us imagine that I am testing reading comprehension and I do so by seeing how many words a student can correctly read out loud in a minute. The student consistently reads right around the same number of words. This is a reliable test, but it is not valid because I do not have any idea whether the student really understands the material.
Thus, a test can be reliable but not valid.