“MAY HAVE”....means who the hell knows...
No, it means that they’re not speaking in absolutes.
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.