In lam yanḍaḥ bi-māʾ, *lam* negates the imperfect verb and gives it a past reference with the jussive form. *Naḍḥ* relates to sprinkling, oozing, or noticeable wetness; here it evokes sweat or moisture after running.
Fa-yughsal expresses the further consequence, that the horse was not in need of washing. This is poetic praise through hyperbole, not a literal biological assertion about how all horses sweat.