Is there an existing Python function I can call to do the same thing as pressing the "Auto-fill Materials" button on the "Material Library" node without the popup dialog?
Or do I need to write this myself?

Do you need to? I thought that was kind of an obsolete workflow. Using '*' kind of does the job.

Oh, I didn't realize that was obsolete. That's how I was first shown how to do it. Now that I know I'll use the '*' way. Thanks jsmack.

Indeed, "Auto-fill Materials" was more useful in 18.0, and that's how you most likely have learnt it. In 18.5, the default values for parameters changed, and the globbing asteristk "*" already includes the material VOPs inside, thus authoring the corresponding USD materials.