Yes, I’m sure that the signalmen must have thought their systems were foolproof. As for their motivation, it seems to have been professional pride more than anything else. It was a matter of pride not to fall down on the job, to keep the traffic moving if at all possible. And though no one would have blamed them if that arriving train had been held up briefly at a signal while the other train departed, they would have had to record the delay in a book, and they wanted to keep a clean sheet.
The only recommendation the enquiry made was to install a track circuit between Signal 171 and Number 95 Points to electronically detect the presence of a train. Interlocked with the points and signals, it would prevent those points being moved when a train was anywhere on that stretch of track. I haven’t been able to find out if that was ever done.