It doesn't seem to say in the article and it's not really discussed in these "LEAVING THE CLOUDS!!" articles, but what are these orgs doing for on-prem? Given the broadcom acquisition of vmware, rebuilding massive vsphere clusters like it's 2010 doesn't seem like a good long term play. Are they moving to kubernetes? Some other hypervisor?
At least in the case of 37signals, they went with colocated servers, some type of KVM and their own tool, Kamal, for containerized deployments without the complexity of kubernetes.
You can find one post here with many links at the bottom