NM, finally found it on this page: https://www.postgresql.org/docs/current/sql-expressions.html#SQL-SYNTAX-ARRAY-CONSTRUCTORS

It is also possible to construct an array from the results of a subquery. In this form, the array constructor is written with the key word ARRAY followed by a parenthesized (not bracketed) subquery. For example:

SELECT ARRAY(SELECT oid FROM pg_proc WHERE proname LIKE 'bytea%');

https://github.com/seancorfield/honeysql/issues/404