Technically it is an ethnicity but the South American population does have a larger percent of assimilated natives than continental Spain and a long history of a caste system which put born natives lower in the hierarchy than those born in Spain which could arguably make them a more divergent set of genetic traits being messily correlated. One can argue about the name but "Hispanic" is a workable proxy for the broad brush of 'everything below the continental born caste'.

Of course if you want to talk genetics instead of silly human labels you look at the actual contents of their genome.