The most logical to me is, they registered some AWS IPv4 address for one project. Bill didn't get payed and now another customer has been appointed to the same address but now with totally different content. DNS admins at the government forgot about it and here we are.
This is very obviously just an S3 bucket-name takeover, so no IP address was hijacked (the IP is the same for all S3 eu-west-1 buckets, I am guessing).
