Someone asked me about the following phrase, which describes a band 9 task response in writing task 2:
"presents a fully developed position with relevant, fully extended and well supported ideas"
Is it really possible to "fully extend and support" your ideas in such a short essay? Probably not. If you had time to write 1000 words, I'm sure you could extend and support your ideas more fully.
However, we shouldn't "overthink" this problem. Examiners know that it's impossible to write a truly perfect, fully developed answer in just 250 words. They understand the limitations of the task, and (hopefully!) they use their common sense. Instead of worrying about the precise meaning of the band descriptions, it's best to simply focus on answering the question as well as you can.