How do you understand the statement"true synonymy is non-existent"?

How do you understand the statement"true synonymy is non-existent"?

Absolute true synonyms. also known as complete synonyms, are one type of synonyms. They are words that are identical in meaning in all its aspects, i.e. both in grammatical meaning and lexical meaning. including conceptual and associative meanings. Synonyms of this type are interchangeable in every way. It is observed that absolute synonyms are rare in natural languages and some hold that such synonyms are non-existent.