Airbus teach to arm the approach when cleared for the approach.
My reading of the Airbus reply above isn't so much saying that you must do it at the level segment, rather that you would do it before the FINAL APP point and the level segment is an example of a perfect place for that.
I regularly GNSS RNAV into uncontrolled airports. I see no reason not to do arm it as Check Airman did at 8000' above the airport. If there's no controller to clear you for an approach (which in itself reminds you to arm it) what harm is there in putting it on early whilst you remember it? Much better option than wondering at o dark o'clock where the blue arrow is on the ND and why it's not going into FINAL APP....