That Y chromosome was added by applying same analysis workflow to a different biological source (CORIELL:NA24385, a NIST standard material used in the Genome in a Bottle project). The other chromosomes are all from the CHM13htert line (if you click on the individual chromosomes at your link above, you can scroll down to the "/isolate" feature to see what material the sequence was derived from). There's a long tradition of having a reference assembly be a combination of different individuals. Even the "standard" GRCh38/hg38 reference doesn't represent any single individual.