This is an interesting discussion. Could you open a discussion about this in Quarto repo directly ?
--output-dir is indeed working differently than in rmarkdown::render(). It will work for Project only, and is used to change the default output folder.
This is expected as your Qmd document is in a scripts subfolder, the results will also be in a same name subfolder as it would be expecting for Quarto project like websites or books. This is how it works.
@adomingues t seems you have another use case to change where the output of a single document are stored. Currently, I don't think there is a flag for that in quarto render. This would be worth discussing for a feature request. For now, using post-render script as shared by @Rory_OG would be the way .