The thing is they have Zone 1 go through the Sky Priority lane.
This is what they have on their website:
http://www.delta.com/content/www/en_...-priority.html
Pre-Boarding
Customers needing assistance or additional time to board, including families with car seats or strollers.
Premium Boarding Zone
Delta One™ customers
First Class customers
Diamond Medallion® members
Sky Priority Boarding Zone
Platinum Medallion® members
Gold Medallion® members
Delta Comfort+™ customers
Flying Blue Platinum and Gold members
Virgin Atlantic Flying Club Gold members
Virgin Australia Platinum and Gold members
GOL Smiles Diamond members
SkyTeam® Elite Plus members
Zone 1
Silver Medallion® members
Priority Boarding Trip Extra customers
Gold, Platinum and Reserve Delta SkyMiles® Credit Card members
Flying Blue Silver members
Virgin Australia Velocity Silver members
GOL Smiles Gold members
Sky Team® Elite
Crossover RewardsTM SPG® Platinum members
Capital One World Mastercard® cardholders
Zone 2
Main Cabin
Zone 3
Basic Economy