Functions used by index don't need to be immutable?

Hi,
while researching some blogpost I found that we can make index on
volatile functions.

Tested this code:

CREATE TABLE wiki_docs (
    id INT8 generated always as identity PRIMARY KEY,
    title TEXT,
    body TEXT
);
create function get_tsvector( IN the_row wiki_docs ) returns
tsvector as $$
    select
    setweight( to_tsvector('english', the_row.title), 'A')
    ||
    setweight( to_tsvector('english', the_row.body),
    'B');
$$ language sql;
create index the_magic_gist on wiki_docs using gist( get_tsvector(wiki_docs) );
create index the_magic_gin on wiki_docs using gin( get_tsvector(wiki_docs) );

Ran it in in every Pg from 10 to 19, and it worked.

postgres=# select provolatile from pg_proc where proname = 'get_tsvector';
 provolatile
-------------
 v
(1 row)

Which seems to contradict part of
https://www.postgresql.org/docs/18/sql-createindex.html
which says:

> All functions and operators used in an index definition must be
> “immutable”,

What am I misunderstanding and/or doing wrong?

Best regards,

depesz