I am actually unsure that the above is really true. let me quote the rules first.
One use only. This is USED as a dispel scroll. When used, roll a D6. On 4+ all Remain in play spells are automatically dispelled and the magic phase ends immediately.
This implies that the cube does not WORK as a dispel scroll, just that you use it in the same way. it does not say that it automatically dispels the spell being cast, only that it can potentially end the magic phase (and dispel remain in play spells, so if the spell being cast when you use the cube is a remain in play spell, i would argue that it is dispelled just as if you had used a dispel scroll, but any other spell should be unaffected.)
I believe that many ppl play with the "it works ad a better version of a dispel scroll"-rules, but i would love to see some support for that in the actual rules. To me it seems pretty clear that "this is used as a dispel scroll" only stipulates when you can use the item (when the opponent casts a spell, you can choose to use it instead of trying to dispel [and according to the FAQ you cant use it against a spell cast with IF]) , and not what effect it has.