Although the airline is responsible for serving food, like any food service, staff cannot guarantee its safety because they do not make every component of the meal. Meals are provided to airlines by contracted third-party catering suppliers. Allergen management depends on these food manufacturers and their processes, which the airline does not control.
For example, on an international flight, one meal may be provided by an Australian catering company, while the meal served after a stopover in Singapore might come from a Singaporean catering company, where food is prepared according to Singaporean food labelling laws, which are different from those in Australia.