Is this a sort of unspoken standard? Because personally, I don't like that style for answering such a straightforward question.
hahahah, no, not an unspoken standard. It's a challenge. The part of the application about writing tests whether you can write well, and many people need to be challenged to achieve brevity.
But 150 is not a lot of words...
"briefly elaborate"--- yeah, good call, that is an oxymoron.
Well, I see that you are a language person, so just do it. If I challenged you to do it in 10 words I would expect you to be able to do it.
Water polo? Well... it will be good to include a few different activities and most importantly a theme, a memorable concept.