A warning would have been absolutely sufficient, at most a very short term ban but given there was no harm being done currently and the rules weren't even clear no ban is justified.
I am aware of vote fraud and it's ok that SO warns/suspends if one engages in it. In this case it arguably does. However it needs to be a proportionate response to the action, and not done as retaliation.
I'm also a fairly high reputation SO contributor (in current rankings top 150).
This specific user has a very long history on SO and the rest of the network. You have to assume that this history might have played a part in the decision to suspend instead of only warning.
1 year suspensions are not handed out for first offenses. The guidelines for mods are to give escalating suspensions for 7/30/365 days, so this is most likely at least the third suspension for this user.
This case could be different. Do you know he was suspended before? It makes sense that they'd want to ban him for a year if this is retribution. It all checks out. The email doesn't mention a previous ban which I'd expect to be mentioned if it was a factor.
I am aware of vote fraud and it's ok that SO warns/suspends if one engages in it. In this case it arguably does. However it needs to be a proportionate response to the action, and not done as retaliation.
I'm also a fairly high reputation SO contributor (in current rankings top 150).