The PRC doesn't like dissent abroad. It doesn't like people challenging its consensus. I tend to believe that where governments find themselves—because of economic dependency or because so many are rather diplomatically cowed by the assertiveness of the People's Republic of China—governments are less willing to speak out than parliamentarians can.
IPAC creates a space for parliamentarians to speak out and try to defend the rules-based order, which is under pressure from China. That's why it matters, and that's why China doesn't like it.