Modifying Arrays

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Page: https://www.postgresql.org/docs/15/arrays.html
Description:

According to the official document of PostgreSQL 15, in the section 8.15.4.
Modifying Arrays there is a statement like:

The slice syntaxes with omitted lower-bound and/or upper-bound can be used
too, but only when updating an array value that is not NULL or
zero-dimensional (otherwise, there is no existing subscript limit to
substitute).

This statement is not true for the following statements or I am missing
something?

CREATE TABLE sal_emp (
name text,
pay_by_quarter integer[],
schedule text[][]
);

INSERT INTO sal_emp
VALUES ('Bill',
'{10000, 10000, 10000, 10000}',
'{{"meeting", "lunch"}, {"training", "presentation"}}');

INSERT INTO sal_emp
VALUES ('Carol',
'{20000, 25000, 25000, 25000}',
'{{"breakfast", "consulting"}, {"meeting", "lunch"}}');

INSERT INTO sal_emp
VALUES ('Carolx',
'{20000, 25001, 25002, 25003}',
'{{"breakfastx", "consultingx"}, {"meetingx", "lunchx"}, {"meetingy",
"lunchy"}}');

INSERT INTO sal_emp
VALUES ('Sam',
'{20000, 25001, 25002, 25003}',
'{}');

INSERT INTO sal_emp
VALUES ('Pam',
'{20000, 25001, 25002, 25003}');

SELECT * FROM sal_emp;

UPDATE sal_emp SET schedule[1:2] = '{"asd", "asa"}'
WHERE name = 'Sam';

UPDATE sal_emp SET schedule[1:2] = '{"x", "y"}'
WHERE name = 'Pam';

SELECT * FROM sal_emp;

In order to test it: https://dbfiddle.uk/pHKY32u0

Thanks!