The subtitle is 'A Definitive Survey 1859 to 1966' so there's at least a good argument for loco allocation listings. Technically the SDJR continued as a legal entity until nationisation although the joint committee didn't own any locos and rolling stock after 1930.
Actually, I would agree that the scope of the book is too broad and it would have been better if the cut-off point was 1930. I think the author set himself a task he couldn't possibly have fulfilled, at least in one volume and perhaps not at all.
There are lots of books available for LSWR, SR, MR, LMS, GWR and BR locomotives so it would have been better to include a bibliography for those types. To be fair there is some crossover concerning MR/LMS locos before 1930, but that could be resolved easily.