More like optimization. Signal processing and control theory are basically the same maths with different applications, optimization is a bit different but has some overlap (especially with, e.g. optimal control techniques).
I would definitely agree that optimisation fits the definition in part but I find really only control theory covers that entire field of signal processing, optimisation, and decision making systems.
And importantly, because ML in some amount touches on all of those, control theory tends to fit better as it focuses so heavily on providing a comprehensive framework for reasoning about all of those elements together.