I don't really know what to tell you. The person who suggest the confrontation against Gul'dan is Sylvanas, and she is the one who says she will cover the flank for the Alliance. Even while the Alliance face the brunt of the forces of Gul'dan head on the Horde fall and fail, but beyond that the only thing they are willing to sacrifice is the Alliance to cover their own retreat. There are Horde forces there, but Sylvanas is unwilling to leave them behind, or coordinate a unified retreat. Please see her listed character bio if you need proof for why she retreats completely:
The only possible interpretation of the events at the Broken Shore is a display of embarrassment for the Horde. Vol'jin is weak and cowardly. Sylvanas is weak and cowardly. Thrall is weak and cowardly. These are facts, and the Alliance are the heroes that stand to fight and sacrifice themselves for the greater cause of battling the Legion.