The issue is why did Paul speak as to unregenerate men of flesh using spiritual milk is they could not understand it?
1 Cor. 3:1-3 teaches unregenerate men of flesh can understand spiritual milk.
Men of flesh and the natural man are lost unregenerate sinners who have the limited spiritual ability to understand and respond to "spiritual milk" which includes the elementary things of the gospel. "The things" (1 Cor. 2:14) contextually refers to spiritual solid food (meat).