It is highly unlikely that there is a misdiagnosis in most cases.
I suspect that the rise in cases of autism and ADHD are likely owing to a mix of the following:
A greater awareness at the "milder" end of the spectrum. When I was at school in the 60s/70s there were children in my classes that I look back on and now think may have been autistic etc but were not recognised as such. They had problems socially and showed differences in the way they spoke and their approach to life and we're at a disadvantage. I can remember one girl at my grammar school getting in to terrible trouble for having an alarm clock in her bag (which went off accidently) she told the teacher that she had it because the ticking noise helped her concentrate (which didn't go down well!). Having taught students with a range of neurodiversities, I can now see that there were other things in her demeanour, the way she spoke etc that could indicate neurodiversity.
A massive increase in the survival of premature babies. Of course not every child born prematurely has autism/ADHD and it is possible for term babies with no birth complications to be neurodiverse but I wouldn't be at all surprised if prematurity can cause subtle brain differences which sometimes result in neurodiversity and that this would account for some of the increase in children being diagnosed. This may also explain why with some identical twins, one is diagnosed autistic and not the other.
Epigenetic effects. I'm less certain about this one but again I wouldn't be surprised if exposure to modern chemicals and pollutants especially from plastics hasn't triggered some epigenetic changes that increase incidents of neurodiversity.
Our world is a changed one in many ways from even 50 years ago so we just can't say "There never used to be this number of neurodiverse children so they must be being misdiagnosed".