Actually, not quite correct @LiteralLunatic CBS and CBD are NOT the same thing. I refer you to the PSP Association:
Although CBD and CBS are often used interchangeably, they do mean different things, which can get confusing. CBD is a pathology, the thing that is causing the disease in the brain. CBS is a syndrome which is a collection of symptoms. CBD in the brain can show in life as different diseases in life, but most people diagnosed with CBS have CBD causing it, which is why they are used interchangeably.
Therefore, a patient could present with CBS symptoms yet not have CBS. Of course, it's all speculation. I don't think anyone is saying the letters are proof of anything. Just that they raise valid questions from available material supplied by RW. The 2015 letter is the first time a tentative diagnosis is given (as far as evidence allows), and that diagnosis is essentially something that resembles those of mild CBS. Meanwhile, the TSP account two years prior in 2013 is terminal CBD. The 2015 letter also appears to have a very detailed grasp of Tim's clinical history, so one would expect earlier diagnoses of CBD would be mentioned. The letter raises doubts, that's all. One day we might find an answer.