Here's a way that preserves ordering. It seems way to complicated to me, I'm hoping there is a more elegant way someone else can provide in a subsequent comment.

%sql
;WITH Blah(col1, i) as
(
          SELECT 'abc',1
    UNION SELECT 'def',2
    UNION SELECT 'ghi',3
)
SELECT 
    array_join(collect_set(col1), ',') as combined_string
FROM Blah;
--ghi,def,abc

---------------------

--but what if you want to preserve ordering?
;WITH Blah(col1, i) as
(
          SELECT 'abc',1
    UNION SELECT 'def',2
    UNION SELECT 'ghi',3
)
SELECT 
    ARRAY_JOIN
    (
        TRANSFORM
        (
            ARRAY_SORT
            (
                ARRAY_AGG( (col1, i) ),
                (left, right) -> CASE WHEN left.i < right.i THEN -1 WHEN left.i > right.i THEN 1 ELSE 0 END
            ),
            x -> x.col1
        ),
        ','
    ) as combined_string
FROM Blah;
--abc,def,ghi