In "1 Introduction", the first paragraph says:
    The [XQuery 3.0 Requirements] states that "The XML Query Language
    MAY have more than one syntax binding. One query language syntax
    MUST be convenient for humans to read and write. One query language
    syntax MUST be expressed in XML in a way that reflects the underlying
    structure of the query."

But the XQuery 3.0 Requirements don't say this (or anything similar). Instead, I think you have to go back to the original XQuery requirements to find something along those lines (though not with exactly that wording).

The corresponding problem also occurs in the CR for XQueryX 3.1.