Ask yourself did you deliberately leave it out or did it not occur to you use it? These exercises are giving practice in participles and yet you wrote a sentence without one.
Yes you do need to use the participle here.
Does Greek prefer a subordinate clause before the main clause
Greek word order is very flexible. You have seen examples in M.'s text where the participle clause comes second but here he puts it first. I confess I dont know why except that putting “ὡς οὐ πλούσιοι ὄντες” first emphasises it. Perhaps he wrote the answer in that way to remind you that word order is flexible?