The optimal workflow is to add only the tasks you are committed to completing today. The app is designed for task completion, not endless organization and neglect. So not integrating with the Reminders app is an intentional choice.
All todo apps already provide a way to have a Today list. What your app brings to the table is the widget. But if that isn’t interoperable with other task management, then it’s a poor value proposition. If you’d make it interoperable, however, then it’s something I would pay for.
Even if I wanted to integrate with Reminders, it wouldn't work that well. The app would have to be running to synchronoize with Reminders. This means the widget would not update if you made changes in the Reminders app and One Task was not running.
Yes, there will be a certain delay for the widget to update, that’s not perfect, but you can always have it update with a user interaction.