Notebook are a bit different. Best you can find as information are
They are completly tied to the IDE and compare to other format, the document is not really knitted.
The Notebook format allow you to run chunks in the IDE and preview the document at the same time. The notebook is not "Knit" but preview, that means a html document containing the text and the evaluated chunks (in a new notebook document with no evaluated chunk yet, if you preview you only have the code chunk without result)
However, I don't think currently a rendering of the document using knitr is happening. So the knitr option is not taken into account.
Try to run the document above chunk by chunk and you'll see the options is not taken into account. I believe this is not yet supported by rstudio.