For this screening check to be worthwhile, it needs to be 'good enough'.
It needs to indicate to schools if their phonics teaching is not up to scratch and if some teachers are still using 'mixed methods' despite evidence to indicate that these fail 1 in 5 children.
This alone could help literally thousands of children who currently do not learn to read well to be taught to read better, by forcing teachers to use the best available method to teach reading.
Even in the school where phonics is taught well, it will pick up a significant proportion of those children who have difficulty decoding and where prompt intervention could prevent ongoing problems.
In this context, a few 'interesting false positives' in the form of able readers is neither here nor there. Even those false positives give indications of next steps in reading - if a child always approaches an unknown word expecting it to 'make sense' in terms of their existing vocabulary, then that limits their ability to acquire new 'real words' from their reading, and that would be worth addressing.
Just because the screening check is not perfect for all children (which actually in Lily's DS's case doesn't really happen - he will pass, as he achieves the pass mark, and his check throws up some interesting extra information about his reading approach - where is the problem?) does not mean it is not good enough for its main purpose for the vast majority of children.
That would be like me saying 'DS worked out the phonics code independently before starting school, so phonics does not need to be taught in school as able children can work it out independently' - I can see beyond the needs of my own child to the needs of the vast majority.