Perhaps the cast members are right, but the Disneyworld website refers to "length of stay":
"When staying at
Disney Resort hotels or other select hotels, Guests can purchase Lightning Lane passes—and choose experiences and arrival windows—starting 7 days before the first day of their Resort stay, for their length of stay (up to 14 days)."
If I had an onsite/offsite split stay that started onsite, then quickly went offsite for a longer stay, I would not be confident that I could book for the full length of my ticket. On the other hand, I think those who are splitting their stays across two or more Disney resorts will be able to book for the full length of the split stay.