Tony Scott had studied at Juilliard and also with composer Stephan Wolpe so I'm inclined to think John Lewis had a good reason to use him.
The Tony Scott official site mentions that
Lewis and Shuller also lead, in November (1955), the concert series Jazz at Town Hall produced by Monte Kay and Pete Kameron in which the Modern Jazz Quartet, with J.J. Johnson (tb), Lucky Thompson(ts), Tony Scott (cl) was supplemented for a large part of the adventurous program by a woodwind chamber orchestra. The program included classical and atonal music by J. Johnson, G. Schuller, J. Lewis, and Luigi Nono.
John Lewis must have been satisfied with Tony Scott's contribution to hire him a few months after the recording session!