If there is no direct object, you may put the indirect object in the accusative case, so I don't see why this was rejected. For example, the following are both correct:
Mi instruas al vi Esperanton.
Mi instruas vin pri Esperanto.
What is incorrect, however, is:
Mi instruas vin Esperanton.*
Technically you're right, but I would not recommend to use accusative for indirect objects. It leads to misunderstandings and you might get mess up your feeling for verb syntaxes.
As I said, technically you are right. Yet, for the sake of keeping good habits, I just recommend not to do it but stick to suitable prepositions. Especially as a beginner or intermediate learner.
Pri la gramatiko: por iel ne vere havas sencon laŭ mi. Krome, pri + infinitivo estas gramatike malĝusta. Mi dirus: “Mi longetempe serĉis manieron (por) paroli kun programistoj pri (la) sencimigo de programoj.”