In Cathay Pacific, all flight crew, cabin crew, ground crew ... have just been advised that they need to file tax, if they fly to or work in the US.
Advised by whom? Was it an official letter? Was it a public announcement? If not, it's likely a bad joke, and I suggest you find the source. Meanwhile, the best way is to ask a direct question. NOT here. If you don't have a union representative, contact the company by sending an official inquiry to the Chief Financial Officer or General Counsel, or both. Their email addresses should be on the website. If it's official, they'll confirm it. If not, they'll rebut it. But in any case, they'll know there is a problem and they need to exaplain it to their people.
Personally, I think it's a bunch of crap.