I think that is a pretty common stereotype, but it's not always a case. A straight A GCSE student might just prefer to go to college rather than sixth form, simply because college offers a wider range of courses.
My 12 year old is really smart (think Sheldon Cooper in a way, only less socially awkward), and I have absolutely no doubt that he'll achieve very high grades for his GCSEs. Academically he could easily go on to do A Levels, but he's hugely into programming and 3D game art, and he's known from very early on that he wants to be a games developer. Yeah he could go on to do an A Level in Computer Science (and choose two other A Level subjects that he couldn't care less about), and he'd get great grades I'm sure, but far more interesting to him would be a BTEC in Games Development, or Digital Games Development and Design.
Obviously we're not at a stage yet of him needing to decide on higher education courses, and if this government has its way by the time he reaches sixth form all BTECs will be scrapped in favour of T Levels anyway (a stupid move if you ask me), but it just highlights that the stereotype doesn't fit everyone.
My eldest two kids fit the stereotype. My Year 11 is fairly academic and wants to go onto sixth form to study A Levels. My Year 10 struggles academically and wants to go on to college to study a Level 3 Extended BTEC.
The only truth in it really is that you do pretty much need reasonably good grades to do A Levels, but even then you don't need all 7s-9s, as many sixth forms only require 5s or 6s for entry. Most middle-of-the-road students can get in to do A Levels.