I believe this is the use of buscar in the imperative not the subjunctive. I think this is the informal (negative) command form. Many forms are the same, but are not from the same parts of grammar. Subjunctive mood and the imperative mood (commands) are not the same.
The second paragraph was a copy/paste from the mini quiz. Sorry it came out so strange when saving the question/statement about the lesson.
Any negative command for informal or formal you, or any other pronoun actually, will always be conjugated in the present subjunctive form. Some people call it "negative imperative" but that results in the conjugation of the present subjunctive. This lesson is about conjugation of this tense; that's why the hints state: "conjugate x form of y verb in El Presente de Subjuntivo", and we don't mention the word "command". Not all questions in this lesson are in a command form, as the present subjunctive is used in different situations.
Hope this clarifies your doubt.
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