More likely the have _something_ which runs on Azure. Fortune 500s are, pretty much by definition, quite large - and probably have tons of departments and sub departments. And at least one of those departments probably has a task of trying out new things, like Azure, by running something on it.
What surprises me is that nearly 20% of Fortune 500s _don't_ have something running on Azure.
(I wonder what percentage "run on" Amazon)