I believe they thought there was assistance on the way. INTERVIEW THE SURVIVORS!
Yes, interview the survivors. But target designation on a dynamic battlefield with many moving targets yeilds position data that is HIGHLY perishable --measurable in a very few minutes or more commonly in seconds.
Like you I believe they did have faith that some help would arrive at SOME point, but that still absolutely does not at all explain why they would be using a laser target designator, especially as they fought off DOZENS of attackers:
Using it has the effect of pointing back to your own position --something that SEALs all know.
Their use of this tool indicates they believed or were lead to believe there was an immediate source of help in-bound or on the ground, nearby.