ACT starts from an uncomfortable observation: the effort to escape an unpleasant emotion strengthens it. The harder you try not to think about something, the more you think about it. The more you avoid anxiety, the more your life narrows around it.
So ACT changes the question. Not "how do I get rid of anxiety?", but "what life do I want, and what am I willing to carry in order to have it?". An unpleasant thought is no longer a problem to solve but background noise you learn to carry while walking in the direction that matters.
The difference from CBT is real, even though both are "behavioural". CBT asks whether the thought is true. ACT asks whether it is useful for you to fight it. A thought can be perfectly true and completely unhelpful to pursue.