Joab means if David did not go and speak kindly to his soldiers, they would all leave David. This can be stated positivley. Alternate translation: "only if you go will any of your men remain with you" or "if you do not go, all of them will leave you" (See: [[rc://en/ta/man/translate/figs-doublenegatives]])
This refers to them remaining loyal to him. Alternate translation: "not one man will remain loyal to you" (See: [[rc://en/ta/man/translate/figs-idiom]])