The idea is that we've separated running computations into pluggable "engines" (whereas in R Markdown everything was hard-coded to use R/Knitr). In Quarto we can use Knitr, or Jupyter, or Observable JS (and can add additional engines in the future). R Markdown was a tool created exclusively for R users whereas Quarto is for users of any language that want to create reproducible documents with Pandoc/markdown.