If you're used to going to a hot, miserable, inhospitable environment (Burning Man) where the guidance for water is 5.7L per person per
24 hours, then bringing 8L (4L each) for a
4hr hike might have felt excessive. That said, it's possible they had the Camelback and some Nalgene bottles and other sources of liquid (juicebox, applesauce, etc. for the baby). LE has not disclosed everything they had in the backpack.
I talked to my BF who is an experienced hiker. Prior to the pandemic, he would go out on all day, very strenuous hikes every weekend. He usually brings 5-6L, but said he hardly ever drinks more than 3L. He said he ran out of water once, in 2019, when the return leg of a hike he had done many times before (Mt. Baldy) was much hotter than had been predicted in the weather report. (He was with friends who shared water.) He also told me that he has had to share his water with random people multiple times on trails.
All this to say, I think even the most experienced hiker can make a mistake. Underestimate how much water is needed, overestimate their own ability. There but for the grace of god, etc.