That is my understanding -- WDW package offers are typically valid for length of reservation as long as one arrives within the stated window. Unfortunately the link doesn't seem to work right now -- probably next week it will.
This is the small print on the offer:
The number of packages allocated for this offer is limited. Length of stay requirements may apply. Everyone in the same room must be on the same package. If reserving by phone, must mention the offer when booking. Must use a valid Disney Visa Card or Disney Rewards Redemption Card to pay the deposit. Disney Visa Cardmember must stay in room. Offer is non-transferable. Additional per-adult charges may apply if there are more than 2 adults per room at Disney Value, Moderate and Deluxe Resorts, and Studios at Disney Deluxe Villa Resorts. Advance package reservations required. Cannot be combined with any other discount or promotion. Theme Park tickets are valid for admission beginning on date of check-in and must be used within a limited number of days, depending on length of stay and ticket. Offer excludes the following room types:
Campsites at Disney's Fort Wilderness Resort,
Treehouse Villas at Disney's Saratoga Springs Resort,
Cabins at Copper Creek Villas & Cabins at Disney's Wilderness Lodge,
Bungalows at Disney's Polynesian Villas & Bungalows, Deluxe Studio - Club Level, 1 Bedroom Villa - Club Level, and 2 Bedroom Lock-Off Villa - Club Level at
Disney's Animal Kingdom Villas - Jambo House, 3-bedroom Grand Villas, and all Moderate and Deluxe Club Level & Suites. Specific number of meals and snacks are determined by the number of nights included in the package stay. Meals and snacks can be redeemed at any time during your stay. Unused meals and snacks will roll over day to day and expire at midnight on day of checkout. Meals and snacks are nontransferable between party members. For the
Disney Dining Plan, some Table-Service restaurants may have limited or no availability at time of package purchase. Learn more at
DisneyRewards.com/DineOffer.