The EU is not a national legislature, they do not have unlimited competence to legislate. If they take anti-monopoly action against a company which is outside t

The EU is not a national legislature, they do not have unlimited competence to legislate. If they take anti-monopoly action against a company which is outside their powers set out in Article 102 TFEU, the company can get it overturned. If they legislate in an area outside their powers, nation states (or an individual who's directly and individually concerned by the law) can get it overturned. This isn't theoretical - there's a lot of litigation in front of the CJEU, and things getting overturned is not unusual.