In applying the verse, the textbook takes quruʾ to mean menstrual periods and states that a divorce during a state of purity leads to a waiting period of three complete menstrual periods. That is the legal interpretation presented in the source.
Two questions must be kept distinct:
How many? The word thalathah gives the determinate number.
What does quruʾ mean? Whether it refers to menstrual periods or periods of purity is a recognized lexical and juristic disagreement.
Certainty about the numerical word does not by itself eliminate every interpretive dispute in the passage.