Yes, technically it does retain this possibility. However, what's important to consider is that heat and light are more primary needs than smoking meat. If the burning of lichen and grass was the best option for heat and light, then it is very possible, if not even likely, that the fires made in these fireplaces were not used for smoking meat but rather for used for heat and light.
Keep in mind, a weaken question does not require the argument to be destroyed or the conclusion to now be impossible. It just requires that the argument is hurt in some way. And in this case, it is by bringing about another (more important) use of the fires other than the one advocated in the argument. Without heat or light, a human would freeze in the dark. With heat and light, they can continually hunt without preserving meat.
I hope this helps. Please let me know if you have any further questions.