No, leaked official cables of the secret designation is not the same thing as an anonymous source. The secret designation only denotes that these cables were intended for people with a clearance level of secret (as opposed to say No foreign, Top Secret, etc). The cables authenticity leave no doubt and have been thoroughly investigated by journalists.

You can read these cables for yourself on WikiLeaks (I don't know why the telegraph wouldn't simply link to them).

Here's one for example:

https://wikileaks.org/plusd/cables/89BEIJING18828_a.html

EYEWITNESS ACCOUNT OF EVENTS AT TIANANMEN SQUARE. ALTHOUGH THEIR ACCOUNT GENERALLY FOLLOWS THOSE PREVIOUSLY REPORTED, THEIR UNIQUE EXPERIENCES PROVIDE ADDITIONAL INSIGHT AND CORROBORATION OF EVENTS IN THE SQUARE. THEY WERE ABLE TO ENTER AND LEAVE THE SQUARE SEVERAL TIMES AND WERE NOT HARASSED BY TROOPS. REMAINING WITH STUDENTS BY THE MONUMENT TO THE PEOPLE'S HEROES UNTIL THE FINAL WITHDRAWAL, THE DIPLOMAT SAID THERE WERE NO MASS SHOOTINGS OF STUDENTS IN THE SQUARE OR AT THE MONUMENT. END SUMMARY.