My reading of the behavioral genetics literature is that high intelligence being driven by rare autism variants is looking unlikely. DeFries-Fulker extremes analyses like "Thinking positively: The genetics of high intelligence", Shakeshaft et al 2015 aren't consistent with the (relatively) high end being due to rare variants (but are consistent with the low end being due to rare variants) and current attempts to find rare variants enriched in the very high IQ with large effect sizes have turned up nothing: "A genome-wide analysis of putative functional and exonic variation associated with extremely high intelligence", Spain et al 2015. There is also an autism heritability observed in the GCTAs/LD score regression using only common SNPs (>=1% population frequency), along with a positive autism/intelligence genetic correlation, which undermines that idea.

My speculation at this point is that Spearman's law of diminishing returns is - based on all the genetic correlations with intelligence which have piled up and the current trends in brain imaging studies finding brain volume/thickness & global connectivity & white-matter integrity & connection speed to be the best predictors of intelligence - is due to intelligence reflecting a bottleneck between all the regions of the brain communicating to solve problems and that as the global communication becomes closer to optimal due to better health & development, individual specialized brain regions start to become the bottleneck to higher performance and shrinking the g factor.