Most likely it is because the resistance profile of bacteria in a species antibiotic behaves both ways depending on the expression of the antibiotic resistance.
Moxifloxacin is, so far, the most active antibiotic towards both the extracellular and intracellular forms of L. monocytogenes. Therefore, animal trials with moxifloxacin for eradication of L. monocytogenes appear justified.