While Ubuntu did build on Debian testing/unstable, they did invest in building the GUI on top of everything, paying salaries for a few Debian developers.

With a very slim team (I am guessing 15-30 in the first couple of years), they picked Python as the go to language and invested heavily in development tooling making it possible for them to innovate and pivot quickly. Yes, they grew to a mid size company of 500-1000 over time, but also expanded into many different areas.

Perhaps one can also make a case for them effectively starting and killing a number of projects akin to Google, except they usually made them open source, and some live on as volunteer efforts (eg. ubuntu touch).