Was this prostate orgasm or just ejaculatory orgasm ?

Sounds like you had P-waves. Never had a Super O but I don't think you had prostate orgasm. Should be more intense than 20-30% of normal ejaculatory orgasm.

Is it common to reach ejaculatory orgasm by prostate stimulation without using hands?

HFWO's (Hands Free Wet Orgasms) while using an Aneros are not really common but they are not so rare as to be considered unusual or abnormal. A few men have found this to be normal for them, it is just something that some men are more susceptable to than others. I don't believe it is anything to be concerned about.

My theory is that it was not prostate orgasm because the intensity of the orgasm was maybe like 20-30% of normal ejaculatory orgasm.

Orgasms can vary widely in intensity from"meh" to "OMG", see the thread What's an Orgasm? for a little further perspective.

Like I mentioned the cum dripped very slowly, there was very little pressure.

This sounds like a description of prostate "milking" which can be very enjoyable in itself. It sounds like you are well on your way to enjoying all the benefits of prostate massage. Welcome to the brotherhood. Good Vibes to You !

Could be pre-cum - that produces a great feeling as it flows. Doubtful it was a HFWO since this is an intense O, like a SuperT. I get them fairly regularly.