In the old days, when I used to use 1000pf caps for analog C band video de-emphasis circuit, Polyester caps gave me a clean flat filter response. Ceramic would smear the picture and the effect was clearly visible both onscreen and on the oscilloscope. However, the motor controller filter you illustrated here does not require such a precision wide-band, consistent smooth slope filter demanding circuit. I guess the engineer behind the circuit just may have had some experience in the past and just said, lets use the good quality caps.
Another thing is that in a high temperature environment, a ceramic cap would drift and it's high frequency impedance completely goes way off in the 47nf range compared to a 47nf Polyester cap.
Again, the circuit you have shown us does not require such demanding performance as it just drives the motors for the faders unless that cap is fighting a high current drive from jfet 'FT800'.