Does anyone have any data about this? Or is this probably a mistaken initial report?... ...(Sometimes, people do not have their facts straight when reporting on an incident.)

They did say this:
"The first message was from 3AM September 11, 2001, five hours before the first attack, and the last, 24 hours later.

"Text pagers are usualy carried by persons operating in an official capacity. Messages in the archive range from Pentagon, FBI, FEMA and New York Police Department exchanges, to computers reporting faults at investment banks inside the World Trade Center

"The archive is a completely objective record of the defining moment of our time. We hope that its entrance into the historical record will lead to a nuanced understanding of how this event led to death, opportunism and war."
http://911.wikileaks.org/

And the report at CBS News, that Joe linked above, speculates:
"How the messages were captured

"The pager logs seem to represent messages transmitted on September 11, 2001 through the networks of Arch Wireless, Metrocall, Skytel, and Weblink Wireless.

"It's not clear how they were obtained in the first place. One possibility is that they were illegally compiled from the records of archived messages maintained by pager companies, and then eventually forwarded to WikiLeaks.

"The second possibility is more likely: Over-the-air interception. Each digital pager is assigned a unique Channel Access Protocol code, or capcode, that tells it to pay attention to what immediately follows. In what amounts to a gentlemen's agreement, no encryption is used, and properly-designed pagers politely ignore what's not addressed to them.

"But an electronic snoop lacking that same sense of etiquette might hook up a sufficiently sophisticated scanner to a Windows computer with lots of disk space -- and record, without much effort, gobs and gobs of over-the-air conversations. "
http://www.cbsnews.com/blogs/2009/11/25/taking_liberties/entry5770280.sh...

"Unencrypted pager traffic has been subject to interception by hackers and others for years — an easy video tutorial made the rounds just last May. But Wikileaks implies this data came from an organized collection effort. “While we are obligated by to protect our sources, it is clear that the information comes from an organization which has been intercepting and archiving national US telecommunications since prior to 9/11.”"
http://www.wired.com/threatlevel/2009/11/wikileaks-pages/

http://911reports.com
http://www.historycommons.org