It is hard to know the real risk with the the little information provided. We would need to know much more (were food contact surfaces positive, how pervasive was the contamination throughout the operation, what trends were seen, what practices were or were not in place, the accessibility of the non-contact areas to being a risk to food, etc), to opine on the appropriateness of this action. Just because Listeria is found in a drain, in itself, would not constitute a risk and can really be considered normal. It seems the supplier was pretty confident in the low risk of the situation.
Business Insider
http://www.businessinsider.com/american-airlines-flights-no-food-after-listeria-outbreak-lax-catering-kitchen-2017-11
American Airlines flights are going without food after listeria found at catering kitchen
Benjamin Zhang
11/1/17