The article does not explicitly say it, but it's clearly a defense against the CLOUD Act (https://en.wikipedia.org/wiki/CLOUD_Act); it all makes sense once you add that missing puzzle piece.

The CLOUD Act conflicts with EU laws like the GDPR (AFAIK, this has been confirmed more than once in EU courts already), which means that EU organizations (which have to follow the GDPR) might not be allowed to use USA-owned cloud services, even when the data is completely hosted within the EU, because the cloud service sysadmins might be forced through the CLOUD Act to break the GDPR. Requiring that all employees with a high level of access have EU citizenship and residency makes it much harder for a USA court to pressure them into breaking these EU laws.