They have unused capacity in their Tennessee datacenter. We don't know everything but the backend of Grok probably runs in the TN datacenter. Grok is not as popular as hoped, so the TN datacenter has unused capacity.
There are other companies besides Google and Anthropic that are considering renting capacity in xaispacex's datacenters.
Global capacity for GPU hosting is tight at the moment. That TN DC is the only one not totally 110 percent already allocated for five years in the world .