Not so. The requirements are that “The licence holder is to take reasonable precautions to prevent unauthorised access”. Which he did not, unless everybody in his household is a licence holder or the cellar was locked and he had sole access to the key. The grounds to revoke a licence is that the licence holder has or is likely to pose a risk to the public. By not taking reasonable precautions to prevent unauthorised access you have posed a risk to the public.
His licence would have been revoked before the court case, which is a separate issue to the firearms offence as far as the police are concerned. Two different departments, firearms licensing and criminal prosecutions.
Even if they believed that there was a likelihood that a revocation will be overturned in appeal, the police will generally go for it as they are “Damned if they do and damned if they don’t”.
Please don’t take my comments as either
schadenfreude or the holier than thou attitude, that some have posted in the past, it is very easy to suffer a brain fart and many of us will have escaped by the skin of our teeth.