It's not like the Reuters reporter wasn't sure because the group withheld Beto's name. Sure, the Reuters article mentions that the group wouldn't confirm (and wouldn't deny) the identity of O'Rourke until after the election.
But, that is beside the point, since the author guessed O'Rourke's identity, and O'Rourke admitted he was a member of the group.
O'Rourke spoke to the reporter on condition that the details of his interview would be withheld until after the election, and the Reuter's reporter agreed. The reporter justifies withholding the info because he was writing a book, and he says he was not looking to get a one time scoop before the election. So, before the election, the reporter knew a lot...he doesn't admit if he knew the story about killing children, though...at least, I haven't found that yet. But, he did do a full interview of Beto for his book well before the election, and at some point learned his pseudonym under which he published. And, the Reuter's reporter seems to have known that O'Rourke had "stolen long distance services" among other things, including being involved with hactivists who published programs for hackers to gain control of people's computers.
https://www.reuters.com/article/us-backstory-usa-politics-orourke-idUSKCN1QX02M
https://twitter.com/josephmenn/status/1106578730904764416