Was it called a "poetry concert"? What is this poetry style called?

If what you witnessed was the same poem being started at intervals so eventually all the poets were reading at the same time, it was NOT a poetry slam. It sounds like someone did exactly what you said: found a poem that had enough resonant words that allowed it to be read "in a round". If they'd added music, you would not have called it poetry in a round, but simply a round...which is the same thing as a musical "round" without the music, right? In this case it sounds like they opted for no music and allowed the beauty and complexity of the words speak for themselves. There is no formal name for this that I'm aware of other than perhaps "poetry in the round".