Jesus quote this to them but it is also quoted by Paul a second time here in Acts 28:26
Acts 28:26 Saying, Go unto this people, and say, Hearing ye shall hear, and shall not understand; and seeing ye shall see, and not perceive: (Mat 13:14, & Isaiah 6:9)
Acts 28:27 For the heart of this people is waxed gross, and their ears are dull of hearing, and their eyes have they closed; lest they should see with their eyes, and hear with their ears, and understand withtheir heart, and should be converted, and I should heal them.
Acts 28:28 Be it known therefore unto you, that the salvation of God is sent unto the Gentiles, and that they will hear it.
But back in Isaiah 6:9 (after the above verse, which is a quote from the same quoted to them by Paul) is followed by what he said about the Gentiles and the salvation of God, and this is a verse (below) falls under that one (back in Isaiah)
Isaiah 6:11 Then said I, Lord, how long? And he answered, Until the cities be wasted without inhabitant, and the houses without man, and the land be utterly desolate,
Isaiah 6:12 And the LORD have removed men far away, and there be a great forsaking in the midst of the land.
Isaiah 6:13 But yet in it shall be a tenth, and it shall return, and shall be eaten: as a teil tree, and as an oak, whose substance is in them, when they cast their leaves: so the holy seed shall be the substance thereof.
So he seems to adress them later that what is posed and declares when the salvation of God is sent to the Gentiles there.