LISTAGG à la Oracle in PostgreSQL

Hello,

I can write a LISTAGG aggregate for:

create table emp(deptno numeric, ename text);

SELECT deptno, LISTAGG(ename, ','::text ORDER BY ename) AS employees 
FROM   emp GROUP BY deptno ORDER BY deptno;

I would like to know if is possible to create an aggregate LISTAGG that 
would work like in Oracle:

SELECT deptno,
        listagg(ename, ',') WITHIN GROUP (ORDER BY ename) AS employees
FROM emp
GROUP BY deptno
ORDER BY deptno;

I failed and IA also failed. Claude says:

It is not possible to exactly replicate listagg(ename, ',') WITHIN GROUP 
(ORDER BY ename) as a custom PostgreSQL aggregate
because PostgreSQL strictly forbids ungrouped columns as direct 
arguments to ordered-set aggregates.

Do you agree ?