Th US government having no knowledge of aliens does not disprove aliens. It is just another data point in the multitude that all trend in that direction. There hasn't been a single data point indicating intelligent alien life ever discovered.
Snowden's revelation is interesting but not surprising. To be fair, it wouldn't be safe to simply assume he is a reliable source. Personally I assume this detail is true because it lines up with every other detail available since the beginning of time, but I can see why someone might be reluctant to take Snowden at his word.
I enjoy reading about the discovery of these planets even if it is a bit scary for future generations. At some point we may discover a viable target and our technology should evolve. Whether it is realistic to ever have a human payload traveling that far is still a question, but it will be answered in time. The universe is likely 100% ours, but it may turn out that we are locked in our little room called Earth. Even then, we would find fascinating ways to expand and explore from afar.