Every source I've read about the project says the city is responsible for overruns. The author goes into the exact details of how this is so, even quoting language from the law and how it might be interpreted.
Nevertheless everything written down and filed with the city attorney respectfully belies Dominic and his faithful echo chamber on this particular issue.
Rather obviously this is going to be subject to litigation, and you didn't have to tell me the above for me to know what the city's position will be ^_^!
A $5 billion cost overrun is a decent possibility. That's about $20K for every worker in Seattle.