Please clarify the topic sentence.

"Nowadays, scientists distinguish among species using methods based on evolutionary descent. Earlier methods were less precise but more colorful. The word “monkey” did not enter the English language until the sixteenth century. Prior to then, the word “ape” was the only common term for primates other than human beings. The difference between apes and human beings was never clear either. If somebody called you an ape, it might not be just a metaphor. In History of Four-Footed Beasts and Serpents, and Insects, published in 1647, Edward Topsell included the satyr and the sphinx among apes — the term included any creature that was almost “human” but not quite. This sort of definition, and not a conventional biological one, must be used when looking back over the old stories of apes and monkeys through the centuries."

The above text is from a book I study. I wonder what is the key sentence of the text as well as the topic.

Does the writer argue that we should use earlier method of defining 'primate' not a conventional biological one? If he does, what does 'this sort of definition' exactly mean?

Thank you very much.