@uriel @stefano worked for years for an ISP/datacenter whose primary datacenter space was in the first level of our office building. We had only one service for the building. It's technically possible to get two, but it would be from the same power company... so when the drunk driver crashed into the transformer and took out our power in winter it would have taken out both anyway. That actually caused a power surge that destroyed our transfer switch which is another problem that having two services wouldn't have solved. We did have diesel backup generators though

We didn't even have diverse entrances into the building for our fiber for a long long time either. But we were definitely a datacenter. (my brother still works there; nothing has really changed except increased bandwidth)

I have never heard of any rules or regulations that require a "datacenter" to have two buildings and independent power. Sounds like something someone made up...