If we had an estimate of the number of in-links and out-links in the inbox, we would have a more accurate sense of the number of people whose communications were actually swept up under this surveillance. The number might be more accurately portrayed as "greater than 49,950 and less than 100,000 people" assuming the typical mailbox contains traffic from 50 people. Not all would be Google's customers, but it would portray the scope more clearly.
If you read the whole article closely you'll find this:
Google noted that the FBI may “obtain ‘the name, address, length of service, and local and long distance toll billing records’ of a subscriber to a wire or electronic communications service. The FBI can’t use NSLs to obtain anything else from Google, such as Gmail content, search queries, YouTube videos or user IP addresses.”
So inboxes are not being opened up for the FBI as part of these requests.
That applies more to the general search warrants they disclose than NSLs.
"Under the Electronic Communications Privacy Act (ECPA) 18 U.S.C. section 2709, the FBI can seek “the name, address, length of service, and local and long distance toll billing records” of a subscriber to a wire or electronic communications service. The FBI can’t use NSLs to obtain anything else from Google, such as Gmail content, search queries, YouTube videos or user IP addresses."
