The BtrBulk class, which was introduced in MySQL 5.7, is by design the exclusive writer to an index. It is therefore unnecessary to acquire the dict_index_t::lock in that code.

Holding the dict_index_t::lock would unnecessarily block other threads (SQL connections and the InnoDB purge threads) from buffering concurrent modifications to being-created secondary indexes.

This was addressed in MySQL 5.7.28 with a change that is still unnecessarily acquiring the dict_index_t::lock in some cases:
Bug #29008298 MYSQLD CRASHES ITSELF WHEN CREATING INDEX