Stoicism, competence, composure, equanimity or level-headedness, can be.

Tranquility, not being a quality of the human mind, but rather a feeling or state of being, cannot be. The dictionary definition of tranquil (free from disturbance) is mutually exclusive with a mind that is actively dealing with concerns of any kind. Because then you are not free of disturbance, are you?

You can remain calm and in control, but if there is force of any kind that you must interact with in any way, you cannot be tranquil.

Can you get there by ignoring any current troubles for a moment, simply not thinking about them for a minute? Yes, but that’s still temporary.

What you are claiming, is like saying silence is the ability to ignore noise.

Or that silence can be “maintained” at a concert. That by refusing to let the music make you dance, you might prevent it being played.

Can you still plug your ears? Sure. But you can’t listen, while doing that.