Fragile X is about 1.4 cases per 10,000 males. Autism is way more common than that, so I wouldn't say the Fragile X accounts for even a single percent of Autism cases. The article saying that it is the most common cause of Autism is definitely weasle-wording, and can only be claimed because most autism is not monogenic in nature. It's almost always a polygenic trait, and therefore doesn't have a single "identified" cause.
Not exactly, currently under the DSM there are a whole bunch of related behavioral phenotypes that are grouped under the ASD umbrella. This includes similar behavioral patterns and processing issues. Yes sometimes it can be polygenic but the reality is that there are a whole bunch of different things that can cause similar behaviors and sensory processing issues. The DSM (and psychology in general) is still pretty shitty in the way that it groups things based on phenotype rather than mechanism
