These jobs had their allotted quotas, per team, giving them above >70% utilization in their logical slice of the cluster. E.g. video processing team gets 20,000 nodes globally. They stack (co-locate) their tasks (interpret: set of processes) however they want.

Granted Twitter operated one big shared Mesos+Aurora offering for everything*, the whole cluster high utilization wouldn't give much flexibility to absorb load, or do reasonable capacity planning (which was an entire org in itself) when you own and operate those machines and data centers. I can't comment much on the 20-30% figure given in MesosCon, it's been more than 5 years since I was last privy to these figures.

I worked for Twitter up until 2017 and when I was there it was much higher than 20-30%, definitely >50%. It's very possibly changed since then, but at least at that point in time Twitter was running Mesos on many thousands of machines.