It’s not a single IP address per data center. Otherwise they’d only be able to make 64k simultaneous egress connections, nor would their scheme of different ip addresses per “geo” and product work.

Edit: based on this part of blog post:

“With a port slice of say 2,048 ports, we can share one IP among 31 servers. However, there is always a possibility of running out of ports. To address this, we've worked hard to be able to reuse the egress ports efficiently.”