What do you base that on? You keep assuming that there's a horde of tens of thousands of barbarians out there when everything we've learned shows that this is not the case. They're not to be underestimated, certainly, but they're still men and they can be killed.
I am assuming so because the forest is very big. Basically, I think we may have killed off most warriors from ONE tribe, but if the forest is so big there can be many other tribes inside. Consider that this old man seems quite calmer and very different from the warriors. So to speak, some sort of "elder". Maybe even some sort of Nature shaman.
I think it strange that, with elders like this, the warriors are raving barbarians. I think we may have found a tribe that preys upon others because they do not follow their Nature spirit. They know about metal forging, they cannot be just fanatics and madmen! They did not just use a metal axe, they gave us a tip on how to make one! So, if their incursions were due to religion, there may be other tribes as well, which are semi-peaceful to each other because they all follow the nature spirit.
You may wonder why I think this is some sort of religious war, but I think it's almost certain. Consider this: they came out of their forest, killed everyone, and went back inside. They take no prisoners, we found heaps of bones with teeth marks. They do not conquer and settle outside. And at this semi prehistoric age there's basically nothing to loot.
So, yes I think the old man was covering his ass, and maybe his tribe is indeed weakened. But this does not mean there are no other tribes inside the forest, similar to this one. And they may get involved if we decide to invade.
Of course, they are just suppositions. But I think them solid enough, and really, they are no more fictional than just saying: yeah surely we have killed most of them and they are just trying to avoid us invading.