SHA-512(...)[:521] would still produce the same vulnerability, there would be 9 unchanging bits (assuming the [:521] would pad the 512 bits to 521). Those 9 guessable bits are enough to recover the key from 60 signatures, as the post explained in detail.

A more interesting question (while we are on the 20/20 hindsight express) is why the dsa_gen_k() function did not include an assert(digest_len <= 512).