Why would you call colocation "building your own data center"? You could call it "colocation" or "renting space in a data center". What are you building? You're racking. Can you say what you mean?
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TFA explain what they're doing, they literally write this:
"In general you have three main choices: Greenfield buildout (...), Cage Colocation (getting a private space inside a provider's datacenter enclosed by mesh walls), or Rack colocation...
We chose the second option"
I don't know how much clearer they can be.