If you click on the Priority Boarding hyperlink, it reads this, which is a bit conflicting:

Via our Delta Sky Priority lane, all passengers in Delta One, First, or Business Class, and all Diamond Medallion members, will be invited to preboard (Zone PREM). All Platinum and Gold Medallion members, SkyTeam ElitePlus customers, and Virgin Australia Velocity Platinum and Gold members are then invited to board (Zone SKY). Silver Medallion members, SkyTeam Elite, and Virgin Australia Velocity Silver members customers can begin boarding after that using the Sky Priority™ boarding lane when traveling in Main Cabin (Zone 1).