«بَلْ أَنْتُمْ قَوْمٌ تَجْهَلُونَ»
Rather, you are a people acting ignorantly.
Antum explicitly addresses the audience. Qawm describes that audience, and the plural subject implied in tajhalun continues to refer to the people addressed. This supplies a clear semantic rationale for second-person agreement.
One should not reject tajhalun merely because qawm is an overt noun. The address perspective prevails over a possible purely formal third-person reading.