Adding a list of extra dependencies in YAML using hyphens in RMD file?

I have the following YAML given to me as an RMD template for use in a course:

---
title: "Chapter 1"
author: 
output:
  pdf_document:
    extra_dependencies:
    - geometry
    - multicol
    - multirow
  html_document:
    df_print: paged
---

However, according to https://rmarkdown.rstudio.com/docs/reference/pdf_document.html, a character vector should be used for the key "extra_dependencies", so how come a hyphened list is allowed?

I don't see any documentation or explanation for this, resources explaining this are welcomed!