EXPLAIN IndexOnlyScan shows disabled when enable_indexonlyscan=on

I was surprised today when I saw that with
enable_indexscan=off
enable_indexonlyscan=on
EXPLAIN prints that the index only scan is disabled:

QUERY PLAN
-----------------------------------------------
Index Only Scan using history_pkey on history
Disabled: true

I wasn't sure if this was expected -- maybe index-only scan is
considered a type of index scan for this purpose. I dug around this
disable_cost thread [1] a bit to see if I could figure out what the
expected behavior is on my own, but I'm still not sure.

Anyway, maybe I'm misunderstanding something.

Here's my repro:

CREATE TABLE history(
id INT PRIMARY KEY GENERATED BY DEFAULT AS IDENTITY,
data TEXT);
INSERT INTO history(data)
select repeat('a', 100) from generate_series(1,10000)i;
VACUUM history;
set enable_seqscan = off;
set enable_indexscan = off;
set enable_bitmapscan = off;
set enable_indexonlyscan = on;
EXPLAIN (costs off) SELECT id from history;

- Melanie

[1] https://www.postgresql.org/message-id/flat/CA%2BTgmoZEg1tyW31t3jxhvDwff29K%3D2C9r6722SuFb%3D3XVKWkow%40mail.gmail.com#402856db473920b9e0193b9f2cc2739b