My dim recollection of reading about this was that no, the aircraft wasn't off course.
But there was a low level conflict going on between US and PRC air units (repeated buzzing, some air to air shootdowns)
It was guessed that the PRC pilots thought that the CPA plane was CAT, and carrying people they didn't like (CAT also flew DC4's) A CAT C47 had been shot down ISTR in 1952 on a covert mission
Case of mistaken identity. Note that PRC paid full compensation asked for, and very promptly. The UK government was noticeably slower to pass on the cash to CX...!