It is a contradiction. Others would argue that they could not use the instrument, but could attend but not sing. That is what he would call unity in diversity. It is wrong to do that which God has not authorized. E. Unity in Diversity is the logical result of the philosophy of subjectivism 1.
Subjectivism teaches that Bible truth is subjective or is determined by the knower.
Truth according to this theory may vary from person to person. But regardless of any person's view, even though it is contradictory of another man's view, it is not wrong, because they have expressed their view of truth according to their own feelings. One person may say that baptism is immersion and another say it is sprinkling and according to this view both would be right. Therefore they conclude that one group could oppose instrumental music and another group approve of it and yet both could be pleasing to God. Even though there are differences in practice and doctrine just ignore them and assume that there is unity.