Which is a fair point, in this study.
All too many reports, however, don't discriminate across age clades, and just count up total Autists, if they specify at all. And they detect massive rises in Autism diagnoses since <whenever>, and you can't tell if adult diagnoses are skewing the results or not.
In this case, that they've accounted for the improvements in diagnosis rates is a positive sign... although I wonder how that "50%" number was arrived at:
However Roithmayr [president of the research and advocacy group Autism Speaks] noted that better and broader diagnosis and higher awareness accounted for only a half of the rise in autism rates, and that the most recent numbers show that there is an autism epidemic in the United States that needs to be addressed.
A lot of Autists simply don't trust Autism Speaks. Most of its money goes to advertising and research into eliminating Autism (which Autists interpret as eliminating the possibility of people like us in the future, at the expense of research into treatments for the disabling symptoms of Autism for people who exist now). There is only one Autist on any of its boards (being John Elder Robison on the Research board, where he is outnumbered fourty-nine to one), and they have produced videos where people talk about killing themselves and their Autistic child and that they only didn't because of the "normal" child at home, in front of that Autistic child. (Just because they may not be able to speak normally doesn't mean they can't understand what you're saying.) Autism Speaks tend, as far as we can see, to be advocating for the parents, not the autistic children (which isn't a problem per se, except that they misrepresent themselves as speaking for the Autists themselves, something which is overwhelmingly not true), and advocating for more resources based on a campaign of fear and loathing of the worst case scenario, and misrepresenting it as the typical case. It would be entirely in character for Autism Speaks to underplay the role of improved diagnosis and overplay the "OMFG EPIDEMIC!!1!", as this plays right into their story of Autism being this Thing which will steal your child in the night and you need to give money to Autism Speaks if you want your child back.
That's not to say it's necessarily wrong, but I do not trust that unsupported statement from that source.