mmmmmmm ... there are many ways to resolve this task without using subqueries. I dont think this is a "best practices".

I also guess the instruction was not clear enough

I'm exploiting a defecting in your testing to pass this. I suggest you improve the test harness.

There is no advantage and this is not at all best practice, it just makes the query less readable.

The sql optimizer is so good that in 99.99999999999% of cases you never have to think about optimization, only readability.