As a logic problem, the answer is straight forward. Heller does not have to define the exact scope of the right in order to prevail, he merely has to show that the exclusivity/narrowness of the DC position is not tenable.
One side says “Only A”
other side says “A and B”
No evidence directly supporting “only A”
Very much supporting A
Much supporting B (negation of only A)
To prevail, the “A and B” argument does not have to show to what extent A and to what extent B, it just has to show that “only A” is negated.