I've only been to Hamina once (nice harbour BTW) but I don't recall if there was a university or technical college there. I'd have thought Helsinki/Espoo, Tampere or Oulu would have been better for communications and near to recruitment grounds.
You are right. Helsinki/Espoo, Tampere or Oulu would have been far better choices from the point of recruitment but this is a large piece of land and probably Google didn't want to let it go.
Besides they must have thought the name Google is enough to get smart people where ever they want.
I assume they are getting a big chunk of local/national/eu grants to support the depressed area and a quick pass on building/environmental permits which you wouldn't get for building it in the capital.
Plus as the article says - this is really the St. Petersburg/Stalingrad/Leningrad data center without the political problems of having to keep renaming it.
There's a data center in Lenoir, NC which they spun as a way of training and employing laid-off furniture / textile mill workers. The fact that this is in a paper mill shut down two years ago makes me think they would try to do something similar.