I'm not clear from the doc which of these scenarios is what they're calling the "leak"
A bunch of people were scraping commonly used urls based on previous OBR reports, in order to report as soon as it was live, as it common with all things of this kind
The mistake was that the URL should have been obfuscated, and only changed to the "clear" URL at publish time, but a plugin was bypassing that and aliasing the "clear" URL to the obfuscated one
We don't actually know that, it's just that the report did hit Reuters pretty swiftly.
Not hard to guess really. Wouldn't they know this was likely and simply choose a less obvious file name?