Diego Garcia hangars not big enough for 777-200.
I guess the theory there is that there was something on the aircraft that the USA did not want to fall into Chinese hands and somehow, aircraft was redirected to the one place nobody would monitor or track it, aside from the US Navy themselves.
Unfortunately, if you can't hide the aircraft, that does fall apart fairly quickly.
It's 'James Bond' plot stuff, nevertheless, one wonders to what lengths USA would go to stop certain technology going to Chinese superpower?
The Chinese deny it entered their airspace, but you would if you were hiding the aircraft at a Chinese airbase, having arrived at night, for whatever reason.