I wonder if Tim was initially given the potential CBD diagnosis (in 2015, not 2013), because most people that have CBS symptoms do suffer from CBD, even though there are other illnesses connected with the symptoms.
However, Tim's illness was very mild and indolent, so much so that by 2019 his consultant wondered if he may be suffering from a rarer illness.
From PSPA website: You may have also heard the term “CBS”, which stands for Corticobasal Syndrome. 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.