Your parenthetical addition to the headline is misleading. The Israelis did NOT pose as Mexicans the officials in Singapore referred to them as Mexicans. That is a huge difference and done for different reasons
Not certain what you're saying. If you're suggesting they just referred to them that way in conversation, it wasn't a secret, perhaps you're right. I've never heard of the incident before this .
However the article states To keep the presence of Israeli soldiers in the city-state a secret, Singapore called them Mexicans. To me this means they meant to keep their presence secret, by referring to them as Mexicans.