I'm keenly aware of their operations as my last company looked into hosting in their facility and I know people who have hosted there too. I have also physically been there. Too bad HN doesn't allow photos, I've got some good ones.
The map shows what I'm talking about very clearly... all of the power is either used for the local Alcoa plants (there are several) or gets exported elsewhere.
There is little usage for that power locally (it is in the middle of nowhere, frozen in the winters and otherwise wasteland surrounded by superfund sites thanks to the smelting).
It also isn't like you can just plug in 115kv line into a 230kv line and call it a day. It would require new construction to send that power somewhere else, which is expensive and hard to justify over someone just paying for the power to stay right there.
I never said anything about 'generating capacity'. The dam, where I also went for a tour, is ~2k mw total, part goes to Canada and the rest to the US. The Massena facility has an allocation of that power.