For anyone else trying to do this, the issue I was running into using that tutorial was the fact that gulp is asynchronous. Meaning the first time my tasks ran via my hook there were no files there to concat. Any time after that though, it always worked because the ng-annotate files were already there when my tasks kicked off.
The solution I found was to run my tasks synchronously (one by one in order) with this npm module.
https://www.npmjs.com/package/run-sequence