From my own experience, I think it's safe to say that Visualization isn't practiced exclusively within the context of Tantra. For instance, I've practiced Visualization (in the context of Vipassana with a Theravada sangha and Zazen with a Soto Zen sangha) but I've never practiced Tantra.
The following amendments (or clarifications) can be made based on feedback so far:
Yes, there will inevitably be plenty of overlap, and in the activities list, some categories were blended due to superficial similarities. The point is not necessarily to separate styles but to compile a list that can be conceptualized as a tree, with various branches and leaves. Some branches are bound to intertwine.
Thank you, Greg, for the feedback. Additional feedback from other practitioners (from all traditions) would be very helpful.