If this plane continued to fly towards the Pakistan coast with no transponder you and I both know that Diego had to track it, we also know the US gov has several means to see anything in that area. The General also indicated in his earliest reports that the pilots were communicating, he did not elaborate on this point. If they did we would have it. You add the Boeing or Rolls Royce data and many Depts of the USA Gov would know where this plane landed. Unlike many on this board I do not think a person, like the General, with his background would go on a national show and be so specific about the location unless he had two or more sources.
I would warn that someone could be using him and feeding him bad info but man I would have to believe this is not his first rodeo.
Time will tell
You are presuming that the plane took the overseas route around the India peninsula. In which case, both Diego Garcia and U.S. Naval assets in the Sea of Arabia would most definitely have tracked the plane.
However, the route along the "northern arc" would have been much easier to traverse undetected. A track along the Burma-Thailand border, around The Hump between Assam and China, then thru the Himalayas of Nepal and Tibet, finally over the Karakorum Range and into Northern Pakistan.
The route offers little in the way of radar surveillance, plus abundant mountains for screening purposes. A map of radar and ATC coverage in the area was posted on an airlines forum and a route that would "thread the needle" was clearly available. Moreover, flying over mountainous country, the 777 could remain at altitude -- greatly improving fuel efficiency.