That is true, but with the #, you could verify all of the info on this exact document, and that is why they blanked it out.
"Letters of verification may be issued in lieu of certified copies (HRS §338-14.3). This document verifies the existence of a birth/death/marriage/divorce certificate on file with the Department of Health and any other information that the applicant provides to be verified relating to the vital event. - link
In other words one could verify all the other information on the purported Obama birth certificate without the certificate number.
I suppose that it is true that if there were some additional information on the birth certificate besides the certificate number and the information already presented, one could not get at that. But it does not appear that certified copies of Hawaii birth certificates contain any additional information that may have possibly been present on original birth certificates. It is not clear if the HSDS would even have records of this.