That's pretty standard practice in journalism. You have a source (e.g. google employee) who you develop a relationship with over years and learn to trust — this is called a trusted source. They tell you something that they have seen first-hand , so you rely on that to be true.

Depending on the standards of the reporter/publication and the impact of the claims being made, you might try to verify the scoop with one or more additional sources.

Considering the guy is self publishing on Twitter and is not currently a reporter at a well respected publication, I'd bet this is just based off of a single source.

The system isn't foolproof but it works pretty well most of the time