If I've understood correctly the one half was disabled on the Spectrum (i.e. the Spectrum would never try to access the defective part and the half that wasn't

If I've understood correctly the one half was disabled on the Spectrum (i.e. the Spectrum would never try to access the defective part and the half that wasn't working had to be identified by Sinclair) - so they really were defective chips, in contrast to chips where the manufacturer deals with the defect.