It's an interesting finding but important to note they are making no claim about causality. In fact, an explicit future question is whether changes in these receptors is present at onset or if it's a result of living with Autism.

Neurons specifically increase / decrease receptor density in response to environmental factors, eg: use of SSRI's. Any excess of neurotransmitter would likely lead to reduction in receptor density as part of the response. So the story can be as much about an excess of neurotransmitter as it is about depletion of the receptor.

Perhaps the main story here is they can use EEGs as a proxy for measuring this effect so they don't need to put people through PET scans to do wider studies.