There's a difference between between requiring some form of screening and not requiring anything. I'm booked on a transatlantic coming up soon and intend to get a legit test from a 3rd party. If my test is positive, I won't go. Being a responsible person, I don't need Royal to tell me that. I'm confident many others others would do the same. Those who do a self-test on the honor system doesn't mean that all of them will cheat. I mean, 5000+ passengers aren't all going show up with a photo of fake self-test results. So requiring some form of proof of testing at least it cuts down the odds vs not requiring anything at all. Especially if you have to defend yourself in a court of law against gross negligence and reckless conduct by a common carrier.
If a bad outbreak occurs, then Royal can point to their testing and vaccination requirements and argue that they did something, enough even, as opposed to doing nothing, though perhaps not exactly the highest standards they can utilize to protect passengers. So to answer your question, the reason why Royal has this policy in my opinion probably has to do with legal liability and avoidance of bad public relations, more than anything else. My "theory" applies just the same and specifically it's an opinion to address comments along the lines as to why Royal still has these covid requirements for transatlantics and the implication in comments that it is arbitrary, not necessary, etc..