It is not an implausible statement. Conservative estimates are that there has been a 1,000-fold increase in diagnoses of autism in children since the turn of the millennium, in the UK and the US at least. 

One in 100,000 children with autism to 1 in 100 children with autism. In 25 years. 

Yet Trump's statement is controversial. So much so that the like of it is rarely made. 

Welker's eyes widened when she heard it. Their whites became clearly visible. We associate the look with a kind of madness. 

And indeed a kind of madness ensued, as Welker eagerly parrotted the party line: 'Scientists say they've gotten better at identifying it.'