You misunderstood because the title is ambiguous. This talks about seven consecutive riffle shuffles ("cut the deck and interleave the piles"): Those are not a

You misunderstood because the title is ambiguous. This talks about seven consecutive riffle shuffles ("cut the deck and interleave the piles"): Those are not a "perfect shuffle" (i.e. same probability for every permutation) by themselves, only after doing them several times consecutively (which is kinda suprising by itself).