It exists as well, but isn't mentioned in the article. The article just cares about single process speed-up.

The other "merge CRCs" solution starts out resembling:

  /* Return the CRC-64 of two sequential blocks, where crc1 is the CRC-64 of the
   first block, crc2 is the CRC-64 of the second block, and len2 is the length
   of the second block. */
  uint64_t crc64_combine(uint64_t crc1, uint64_t crc2, uintmax_t len2)