I guess form W-9 is a new form from IRS. Before IRS asks us to fill out 1040NR if we want to claim tax treaty benefit. W-9 form is obviously a change from that, because it recognizes both resident status and tax treaty.
To answer your question about the statement in W-9, you can use the same wording for form 8233 (eg. I am a student from China on F-1 visa. I am studying toward my PhD. ..... Tax treaty article 20(c) states........)