From the whitepaper, page 41:
Keys are generated in a merkle tree and are very deep within the tree. For instance, Alice pre-generates one million keys, each key being a child of the previous key. Alice allocates which keys to use according to some deterministic manner.
Are those million keys all in a row?
H11 H21 H22 ... Hm1 Hm2 ... Hmn K1 K2 ... Kn
Where m = 20, n = 1,000,000 and Km1 is the parent key of Km2, Km2 is the parent key of Km3 ... as they are BIP32-compliant.
The structure above seems to satisfy the conditions, as the keys are very deep in the tree. But that's exactly the slightly confusing part. Does that mean that keys could also be hashes upper in the tree?