Well, they claim to be children of the Union of Brest which included Ukrainians and Belarusans, use the Byzantine-Slavonic Rite in its Nikonian Recension, and aren't part of the Ukrainian Catholic Metropolia of Poland.
They do not use that name and they don't have any ties with the Belarusian Catholic Church. They claim that they are unique.
Oversight may have been delegated by the Metropolitan Archbishop to his suffragan but the Archbishop of Warsaw remains the ordinary for this parish and the Armenian Catholic parishes in Poland, at least according to Rome.
Pope Benedict XVI made on June 9 2007 Bishop Zbigniew Kiernikowski (an ordinary of the Siedlce diocese, not a suffragan to anyone) an apostolic visitator of the Byzantine-Slavonic Catholic Church in Poland.
As children of the Union of Brest the Ruthenian Recension of the Byzantine Rite is their heritage along with the Ukrainians, Rusyns, Slovaks, Hungarians, and Croats and it is older than the Nikonian Recension they have now adopted.
The mother of the parish is not the Union of Brest but the request of the Orthodox parishioners in 1927 to be transferred under the Pope. They follow the synodical Liturgy.
Your information is a bit incomplete.