According to the SCP wiki:
"Keter-class SCPs are anomalies that are exceedingly difficult to contain consistently or reliably, with containment procedures often being extensive and complex. The Foundation often can't contain these SCPs well due to not having a solid understanding of the anomaly, or lacking the technology to properly contain or counter it. A Keter SCP does not mean the SCP is dangerous, just that it is simply very difficult or costly to contain."
When the only reliable way of containing something is abandoning Earth en masse and finding a new planet and restarting civilization on said new planet, then it's probably fair to count that thing as 'exceedingly difficult to contain consistently or reliably, with containment procedures often being extensive and complex'. After all, a containment procedure that revolves around abandoning Earth and moving every non-Keter class object to another planet surely counts as 'very difficult and very costly'. Keter doesn't actually mean 'not containable at all', just 'very difficult and very costly'.
