Physically going to court or not does not matter. Majority of cases in UK courts are just dealt with on paper. According to law, you did go to court, by sending papers to court.
Now, why would a ccj be regarded as a custodial offence? A custodial offence is simply what it says, an offence with time in custody.
A ccj is basically not even an offence. Its a civil judgement against you. You must declare it as such on the form. If you have satisfied it, then it shouldn't have a negative effect on your naturalisation application. But if you are a serial defaulter who often keeps defaulting on your loans etc, then it could create problems.