The researchers found evidence of human occupation and evidence of the plants. One possibility is that the plant material ended up there without human involvement: animals, wind, etc. Another possibility is that the people brought the plants there.
If the people brought what seem to be non-food plants to the site, then medicines, dye, or yarn are some of the possible uses.
Maybe a week from now, researchers will find the same kind of material in another site that has more evidence about how the plants got there or were used.
If there’s just a big pile of every kind of plant, then they could have just washed in. But there’s no reason to think that’s the case.