Bro, You are right that in essay paper "its all about your command over English language and its all about how u can produce a good writing out of something u virtually don't know about" but it doesn't mean that you write something out of the topic, you have to stick to the concerned topic. If it was only the test of good writing skills then FPSC might ask in paper " Write on any topic of your interest an essay of 4000 words" pir topics dainai ke zarurat he nahe the..
Anyways congrats on your success and all the best for your psychological and in interview.