First, the articles do not “conclude that we should abandon looking for environmental factors.” In fact, I highlight the importance of environmental factors in causing autism. Here is what I wrote (http://www.latimes.com/news/local/autism/la-me-autism-science-sidebar-html,0,5744660.htmlstory ):
“As in many medical conditions, the most likely scenario is that genes create a susceptibility, while environmental factors — which scientists broadly define to include such things as diet, drug use, radiation exposure and stress levels — act as triggers.”
Clearly, it would be very useful to identify these triggers—and to see if such exposures are on the rise.
Second, it was not by mistake that the series did not mention the latest twin study. You correctly note that I wrote about it earlier — and anybody who follows the link you provided will see that the analysis breaking down genetic and environment contributions was not accepted by many other scientists I interviewed. The authors themselves noted that their calculations were subject to a wide margin of error and therefore could have led them to the wrong conclusion.
Interestingly, another study I reported on in August (http://articles.latimes.com/2011/aug/15/health/la-he-autism-20110815) found greater risk than previously thought for children with an autistic sibling -- a risk on par with the concordance rates reported for fraternal twins, which is what one would expect for a disorder with a strong genetic component.