Would they have shielded instances from rest of instances processing data for plus/pro/enterprise customers?
What if the employees building the thing are not US citizens?
Technically, there is no citizenship requirement for FedRAMP.
In reality, at every US Government customer I'm aware of, US Persons (Citizen or PermRes) are the only ones that can deploy code to production. You can have non-USP writing code but it must be reviewed and rolled out by USP.