FWIW as someone who is both a native English speaker and who grew up in a community where 90% of residents went to church every Sunday, the only usage of "ecume

FWIW as someone who is both a native English speaker and who grew up in a community where 90% of residents went to church every Sunday, the only usage of "ecumenical" that I'm familiar with would be "open to all religions" or "including people of all faiths", not Christian only. It's not a super commonly used word, so I'm unsurprised that the definition you were taught is not how it's contemporarily used in middle America. Probably a case where if enough people use a word "wrong" then the "wrong" definition becomes a valid usage.