Biblatex does not care about pandoc options

Publishing Your Results
2

I believe biblatexoptionsis a Pandoc variable, and not an argument of bookdown::pdf_book
You should just change the indentation in the YAML

output:
  bookdown::pdf_book:
    latex_engine: xelatex
    citation_package: biblatex
biblatexoptions: [backend=biber, style=authoryear,citestyle=authoryear]

Some details about this: 3.3 PDF document | R Markdown: The Definitive Guide

Have you tried that already ?