So I normally render my documents using either the knit button or the "Build" button for the whole book--I almost never use the console. The knit button doesn't work in this particular use case as it did before, but I was able to get the single chapter I was looking for when using rmarkdown::render(). So thanks. It is slightly more convenient to just click the button, but I'm glad to have a solution.
A similar and more simple example would have been to ask about rendering slides from within a bookdown project. I just realized today that this poses a similar problem, i.e., I wasn't able to just click "knit" in the slides.Rmd file, even if they're in their own folder within the project. But it also has the same solution with rmarkdown::render().
I also appreciate the advice about using github. I'll consider that in the future. And the link to the talk.
Regards