From an article about it:

Now, it should be stressed that this is a build of The Witcher 4 specifically designed to show off Unreal Engine’s features. Yes, it’s running on a standard PS5, but it’s not necessarily indicative of the finished product.

So that’s like saying “under laboratory conditions it has been demonstrated to work”.

When most games with RT in them deliver that performance on one generation old hardware, then you will have proven the point that for most gamers it has no significant negative impact on performance.