It was Henry’s father who got the dispensation. The marriage of Arthur and Catherine was what most such at their level was, in the day, a marriage of dynasties for political reasons. Henry VII wanted to maintain that, and keep the dowry he had already spent, in the bargain. He had toyed with the idea of taking Catherine as his own wife, but settled on Henry, fils.
The dispensation was likely faulty, in that, while dispensing from the impediment of affinity, it did not take into account the the impediment the justice of public honesty, if the marriage had not been consummated. Which Katherine (and her duenna)had always said was the case. Which leads into a lot of history I will pass over. Henry based his _causa_on neither point, but on the Levintine prohibition, and that the dispensation was thus ultra vires, beyond even a Pope’s dispensation. That is, he asserted that the marriage was invalid, as you say. At the time, the rule was that if a valid marriage was contracted, and consummated, and later a dispensation was sought for some one who would have an impediment to marrying due to affinity (marrying Arthur’s widow, that is), the dispensation need only specifically state that the affinity impediment was dispensed, and the impediment of public honesty was thereby dispensed, implicitly. But, if the marriage was not consummated, as Catherine maintained all along, and as was likely true, then the justice of public honesty must be explicitly dispensed. Julius didn’t do that. And hence there was a good case for Henry. And it had the advantage of saying the original dispensation was faulty, based on inadequate information, not ultra vires. It didn’t poke the Pope in the eye.
Wolsey urged him to take that tack. Hank didn’t, and it didn’t really matter. Given the relationship between Clement and Charles, and Charles and Catherine, and the general state of the political world at the time, no way was Henry going to get a decree of nullity. An Emperor trumps a King. And an emperor controlling a Pope is stronger still. So Henry didn’t get his decree. He got a Church, instead.
Not divorce. Decree of nullity. And there was one. Henry got it from Cranmer, his very own Archbishop. So all was well. Said Henry.