It is not a sect, but a rite within the Catholic Church. Byzantine Rite Catholics submit to the pope and are in full communion with with Roman-rite Catholics.
A rite is a section within the Catholic Church that is allowed to observe non-Roman liturgical and canonical traditions, subject to papal approval.
There are many rites within the Catholic Church, including the Roman rite, the Ambrosian rite, the Mozarabic Rite, various Byzantine rite, the Melkite Rite, the Chaldean Rite, and many others. They are no less Catholic than Roman Catholics, which is short hand for Roman-rite Catholics.