Best of luck with your interview! I would think what you are doing to prepare for the written assessment is all that’s possible. I haven’t been invited for interview for the secondary math course here at Cambridge yet, but I do have one with Oxford on Thursday which has the same type of task(they will email a written task the day before the interview and I’m required to send them a scanned copy of my work before 9 am on the day of interview) , I think In general they are looking to see if you can write neatly with correct grammar and punctuation, whilst also being able to layout your ideas clearly.
May I ask what the article is about that they have gave you?
I have already completed an interview at one provider and have been given a conditional offer. In the interview they asked the obvious questions why you want to teach, how you would promote diversity and inclusion within the classroom and what you would do to address challenging behaviour and what some of the reasons are for why students can be disruptive.