I think the docs mean that postgres expects to be sent a 'month name' that has already been blank-padded to be 9 chars long.
So, if you pad the month before sending it to pg, it works:
openacs4=# select to_date('2001-January -12'::varchar, 'YYYY-Month-DD'::varchar); to_date ------------ 2001-01-12
Although, I agree that I would have expected pg to handle your version (and Oracle does).