Continuing the default-extras vs default-optional-dependencies-keys topic from the previous thread: the optional-dependencies table is defined in the specification as " For optional-dependencies, it is a table where each key specifies an extra and whose value is an array of strings." (emphasis added).
So even though the term “extra” doesn’t currently appear in the syntax of pyproject.toml, it does appear in the existing specification. The only reason it doesn’t appear in the syntax is because that table is named after the values it holds rather than after the keys.
Since folks do need to know that the published groups of optional dependencies are called “extras” when referring to them in a dependency specifier, I think it’s fine to use the shorter field name in this PEP (given that “the keys in the optional dependencies table” is literally one of the definitions of “What are ‘extras’ in Python packaging terms?”).