If the companies knew about the data collection but were prevented from speaking about it due to being served with national security letters, does this admission change what they can talk about? And/or does it indirectly confirm the existence of NSLs?
No. They were only prevented from speaking about how many they received, and still are except in broad buckets. The existence of NSLs and the fact that they were about collecting certain users' data have both been directly confirmed by all parties since NSLs existed.
