If they did, their song would not sound agreeable to us.

They sing outside of scales and it is delightful, but that is not man-made art.
Some Spanish singers give a similar impression, through singing interminable grace notes beyond notation.

Their art is intermediate between the singing of the birds and of man.

It is not a higher art.
In certain quarters they marvel at the progress made in the last thirty years.

The architects of the Fifteenth Century must have reasoned in the same way.