"some guy" is, in this case, a British Ambassador. There's no particular reason for him to fabricate this story (in fact, it has the potential to make his job much tougher), and a proper historian could dig through other communications and see how prone to exaggeration he is. My gut instinct is that the cable is more or less true, in the sense that he's reporting claims from a source he's identified as someone he trusts ("THIS SOURCE HAS PREVIOUSLY PROVED RELIABLE AND WAS CAREFUL TO SEPARATE FACT FROM SPECULATION AND RUMOUR.").

The person providing the information is a bit trickier. Chinese politics have always had a bunch of different factions fighting for influence (despite how Western media portrays it as some Orientalist absolute monarchy). Whoever provided the information was likely trying to undermine some faction or another. Perhaps someone from Foreign Affairs trying to undermine some PLA-affiliated faction or another. That'd be trickier to analyze.