I think you are using two different actions. There is an “Add to List” action, you likely installed from the directory, as discussed in this article, which adds the text of your current draft to a list in another draft.
The app also ships with actions that make the selected text in the current draft into a checkmark list. This is in the default “Editing” action group, and labelled “Toggle Tasks” in the action list, but just looks like a checkbox in the action bar above the keyboard.