First time passenger flight by two pilots?
It is never “the first time”. What he is really saying is that after many hours of flying the aircraft (or an approved simulator) this is the first time doing a revenue flight with passengers. Previous flights would have been training flights.
When a new aircraft type is first introduced to an airline, it can be expected that the entire crew is doing their first revenue flight after training.
What is not expected is that the pilots announce it to the passengers. I would not advise doing that because the passengers might not understand, and it would put doubts in their minds, as it did for you.