I think the traditional definition of ecumenism was the unity of all Christians in the Catholic Church. In recent times (50 years) the definition is more a unity among the Christian churches. If the goal of the inter-faith dialogue is to learn about religious cultures, their religious beliefs, etc., and the focus is not on conversion, then yes, it’s a fools errand. “Hence they could not be saved who, knowing that the Catholic Church was founded as necessary by God through Christ, would refuse either to enter it or to remain in it. (CCC 846)”
God bless**
Is it really a fools errand though in this era when Christianity, both Catholic or Protestant, is under constant attack? It seems to me that we’re all stronger together than apart despite our differences.