This is an interesting point. Some parents (or research) have pointed out that the high attainment GCSE results of the selective school is mainly due to the high selection criteria in their entry process.
On the contrary, to your point, the correlation of the results of 11+ admission aptitude test and GCSE scores may not always be strong. As aptitude tests measure different skills and abilities than GCSEs, at different time.
However, other factors such as motivation, study habits, and personal circumstances can also impact a student's performance on both aptitude tests and GCSEs
It is important to note that aptitude test of the admission process and the extent of their correlation with GCSE scores can vary between individuals. The relative results of the aptitude tests, in my opinion, are still a use proxy to gauge your DCs academic progress or potential today, especially for the children from state schools that had little access to CAT scores and etc.