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.”
I don't know if their stack allows for it, but the number of concurrent connections is limited by unique 5-tuples (which is what the host uses to identify the sockets), not just source ports. There is presumably quite a lot of entropy available in destination IP & port to enable far more than 2048 connections per server. But it's hard to be deterministic about how much, so they might be choosing to ignore this and use the lower bound.
I guess for large data centers they could use multiple IPs and the overall scheme stays essentially the same, but the article seems to strongly imply that it is one single egress IP per DC. The 64k simultaneous connections limitation is only per origin server IP.
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.”