They mention tuning io_capacity and io_capacity_max, which unfortunately the MySQL docs indicate is useful until you click through to see what the parameters do [0]. They control background IO actions like change buffer merges, and in fact will take IO from the main process that needs it for work.
IME with a decently busy (120K QPS) MySQL DB is that you do not need to touch either of these. If you think you do, monitor the time to fill the redo log, and the dirty page percent in the buffer pool. There are probably other parameters you should tune instead.
To be fair they aren't going nuts with them, I've seen worse examples. But I agree with you in principle, it's not necessary, and potentially harmful to overall performance. It also doesn't really belong in a ZFS tuning guide.
IME with a decently busy (120K QPS) MySQL DB is that you do not need to touch either of these. If you think you do, monitor the time to fill the redo log, and the dirty page percent in the buffer pool. There are probably other parameters you should tune instead.
[0] https://dev.mysql.com/doc/refman/8.0/en/innodb-parameters.ht...