Hello! YES! Thank you for this note, it’s a good point. The answer is yes, because while phpBB update, it’s not good that another mysql connection interfere with phpBB db tables inserting new data (if new user register in WP because you allow it for example).
Correct procedure:
disable wp_w3all plugin on WP admin, and disable registration in WordPress.
Re-enable these two options after phpBB has been updated.
Thank to your hint, this point will be added on the Help Install Page as soon.