It is not silly. It could be that wide spread and persistent use of herbs required a certain level of population density and people traveling far to spread the word and learn from others. That is why observing apes doing that was important since it showed that practice could be sustainable among early ancestors of humans.

> It could be that wide spread and persistent use of herbs required a certain level of population density and people traveling far to spread the word and learn from others.

It could be, but that's an assumption. Again, the null hypothesis being posited is "premodern humans didn't do this, unless proven otherwise," which is absolutely an assumption, and one already derived from a distinctly unique mode of human experience, i.e. Urban dweller in a high population. Even at the time that scholars were developing the mental models that have continued today to make the field willingly blind to actual human existence, most humans lived in one ecosystem over a long period of time and passed down oral folklore about medicinal herbs.

By and large, for most of human history, most humans lived in the same bioregion as their ancestors and their ancestors' ancestors. The notion that they didn't know how the plants, animals, and other natural phenomenon operated in their environment - that they were so incurious and stupid as to not gain facility with their land, and so incurious, stupid, and antisocial as to not share this information by default - is absolutely an assumption made by the field, one that's belied by enormous amounts of oral mythological, horticultural, and medicinal tradition among every single culture we know of, including the culture of the people who founded the field, and frankly, one that's led to Anthropology's rather poor historical record vis-a-vis native cultures and racism. It's a practice of willful blindness akin to John Miur cropping natives out of his picture of Yosemite because he wanted to capture the "untrammeled wilderness" and couldn't see what he was really looking at.

We have a lot of archeological evidence that at least from the end of ice age people did travel long distances within short period of time, like several hundreds of miles or more. That implies exchange of information on much greater scale then it was possible with a small group leaving in a designated area. Yet we do not have evidence of such travel before the last ice age . Similarly humans started to live in a big villages only after the last ice age according to existing evidence. And again, bigger group implies faster spread of information.

So assuming, as with other activities, that widespread use of herbs to treat specific illness did not happen before the last ice age was a reasonable null hypothesis.

You’re assuming that there’s a small number of herbs discovered a limited number of times and spread over a large geographic region, as opposed to a large variety of potentially beneficial plants discovered multiple times independently by people who spent literally their entire lives, for generations, interacting with their surroundings. Yes, if someone discovered aspirin, and you want to know if it ever reached another continent, your model is useful; if your question is generally “did humans intentionally consume bioactive plants with knowledge of their effects”, I can pretty confidently say the answer is “yes” for any group of humans who spent even a generation living in the same place. Again, these folks lived primarily forager lifestyle, in which they had to know which plants they could eat or they would literally starve to death. They knew their environment.