Cloudflare has also historically used “datacenter” to refer to their rack deployments.
All that said, for the purpose of the blog post, “building your own datacenter” is misleading.
Even where they do lease wholesale space, you'd be hard pushed to find examples of more than one in a single building. If you count them as Microsoft, Google, AWS then I'm not sure I can think of a single example off the top of my head. Only really possible if you start including players like IBM or Oracle in that list.
That being said, this page amusingly mentions colocation in reference to media destruction: https://learn.microsoft.com/en-us/compliance/assurance/assur...