What do we mean by this?
We don’t mean anything by this, because this is just an opinion of certain idealistic authorities. It is rather a philosophical claim which nobody can proof.
You could extend this thesis that music is not absolute if you can see the musicians or the conductor.
I could claim that absolute music is only the idea, the invention of motifs and themes, only the notation. As soon that it is performed music is narrative, as the performers and the conductor always will tell us more than the written text.
And also the audience, every listener has his own associations and imaginations!
So, shortly, the assumption that program music has less value than absolute music - in my opinion - is nonsense.
Is Beethovens Pastorale less precious than his Symphony No. 5?
Many people have a better approach to music by the help of a story of a play: Die Zauberflöte, Peter and the Wulf, Pictures of an Exhibition, even Handel’s Messiah, Bachs Passions of St. Mathew and St. John have a plot. BurBut the music with or without text remains the same.
The perception is different: If someone listens to narrative music without knowing the background story this music is absolute. But if you have the lyrics of a song, the libretto of an Opera, or a great performance, a perfect transmission ... the music will be complete!
Mind that music is always performed in a social context and all these variables like audience, artists, conductor, room, (concert hall, church, open air) are changing and influencing the perception of music.