This is how I've interpreted the following part:
The Development Team modifies Sprint Backlog throughout the Sprint, and the Sprint Backlog emerges during the Sprint"..."As new work is required, the Development Team adds it to the Sprint Backlog. As work is performed or completed, the estimated remaining work is updated.
- As new work is required = extra tasks for a specific User Story that is needed to complete the User Story.
- As work is performed or completed, the estimated remaining work is updated = when tasks are performed or completed, modify it to which you think it is correct.
Usually in my teams we attend to guestimate User Story relative from each other by using the Fibonacci numbers and subtasks by hours or sometimes nothing.
I’m my opinion it doesn’t really matter whether you use time, complexity, t-shirt sizing or even nothing, in the end it’s all about completing the stories that the team had forecasted. I’ve seen teams without any indication doing extremely well and teams using time, complexity etc. fail miserably.