Strictly (pedantically) speaking, using a backreference turns it from a "regular expression" into a "regex", regexes being a superset of regular expressions that are no longer strictly limited to being regular.

Isn't terminology great?

It's true, not false.

You refer to a misleading overloading of the term when referring to the stack machines needed to realize 'regular expressions with backreference memories', which expands the language class to which they correspond.

Citation : https://www.amazon.com/Introduction-Automata-Languages-Compu...

Context matters, in the context of the original post that was not how they used the term.