You jumped on me on a previous occasion for using the term "Christian" to refer to non-Catholics. Here, I see Pius XI doing the same thing.
Therefore I expect that in future, there will be no quibbling from you when I do likewise. What's good enough for Pius XI is good enough for me.
When we communicate to the World, we have to use their language, what they want to be called, what they call themselves. But let's not get carried away with it. I call them non-Catholics, or Protestants or Eastern Orthodox. Avoiding using Christian unless it's unavoidable. Remember all the Church quotes from the Fathers to Pius IX that I posted.