Question: How is pgvector applied in this way different from and maybe superior to the PostgreSQL cube extension?

https://www.postgresql.org/docs/current/cube.html

The cube datatype also stores arrays of numbers (cubes or vectors, same thing) and has functions/operators for computing distance. Granted, they're slightly different, with no direct substitute for cosine distance, but on the unit sphere the cosine distance, inner product, and Euclidean distance have direct correspondences.