The parent is being slightly confusing by bringing temperament into the discussion. They mean that the 12 notes are evenly spaced on an evenly tempered instrume

The parent is being slightly confusing by bringing temperament into the discussion. They mean that the 12 notes are evenly spaced on an evenly tempered instrument, which means of course that you don't get the 'perfect' fourth or fifth you would get on a naturally tempered instrument (the frequencies are very slightly off). Nearly all music we hear now is evenly tempered, but that is a pretty recent innovation.