Many enterprises will only use RHEL (tooling, support, market maturity etc), hence 'enterprise-grade'.
...and enterprise software vendors (eg - Oracle) officially support their software running on RHEL. If you're paying 6 figures for a software license, the difference between APT and RPM fades quickly.
What tooling and support are you talking about that RHEL has that is not available for Ubuntu?
I guess the point here is that Enterprise Linux represents a significant share of the market and if Docker doesn't work on it then it can hardly be a standard. With Red Hat behind this project it's a significant boost to Docker becoming a standard around Linux containers. Does that make sense?