I don't find that link convincing. For one thing, the premise is assumed in the question. Secondly, when it says "they also include ()[] as operators" is implies it is talking about language-specific terminology rather than a general term.

WP describes: https://en.wikipedia.org/wiki/Operator_(computer_programming...

> Occasionally parts of a language may be described as "matchfix" or "circumfix" operators, either to simplify the language's description or implementation.

Bu then, the term "operator" seems to also have mathematical roots: https://en.wikipedia.org/wiki/Operator_(mathematics)

The relevant part seems to be the definition refering to syntax, or denotation:

> Operator is also used for denoting the symbol of a mathematical operation. This is related with the meaning of "operator" in computer programming

Now, whether "the symbol" implies a singular symbol, and excludes delimiter symbols might be a matter of semantics ;-)