I was waiting for the professionals to weigh in, but I'm not seeing that happening.
It's my understanding that QSI were the first in mobile DCC sound decoders. The initial design didn't have that external capacitor. The results were lights and sounds 'blinking' as the locomotive moved down the track, or worse, stalled. The external capacitor included in your Titan U is installed only in the part of the circuit that keeps the lights and sounds alive during operation, at least temporarily.
Adding an additional 'keep alive' high value capacitor, installed in such a way that the whole board, including motor drive circuit, will remain energized, will take care of the lights and sounds as well. The small external capacitor is not absolutely necessary.
Easy to test. Simply clip one of the wires and run the loco.