I think it’s probably more likely this:
core/LimitLoginAttempts.php:848
if ( $this->get_option('notify_email_after') > $this->get_option( 'allowed_lockouts' ) )
$this->update_option( 'notify_email_after', $this->get_option( 'allowed_lockouts' ) );
I was trying to set it to notify me after 5 lockouts, while “N lockouts increase lockout time to…” was set at 2. Think I’ve answered my own question there, although it’s not immediately obvious why notify_email_after can’t be > allowed_lockouts?