The US Government did not say aliens exist. A former military officer said “nonhuman” biological material was in one of the craft they downed. That could mean someone put a monkey or a dog in a test vehicle, it could mean a bird flew into the thing.
Even if he was saying it’s aliens, testifying to congress is not the same thing as the government itself claiming aliens exist. They are reporting to the government, not making statements on behalf of the government.