In the Philippines, we have different kinds of sinigang. We have sinigang na baboy, sinigang na bangus (milkfish), sinigang na maya maya (fish). So, the "na" there means it pertains to specifically the "baboy" and not other kinds of sinigang.
We also cooked sinigang in different ways. We cooked it via santol, bayabas, or any thing that is sour to our tastes. So for your phrase, the sinigang na baboy is cooked in "bayabas".
For me, I like the santol better! Haha. I'm actually craving right now. Let me know if you have more questions.