To me a more natural interpretation from a mathematical POV would use log-odds. So if the author is at the 90% mark, someone 10 times as smart occurs at the frequency of around 1 in 3 billion.
But yeah. In context, your way makes more sense, if only because it's more charitable.
Recently I stumbled upon Richard Carrier's essay "Are We Doomed" (June 5, 2009), when asked to comment about the Singularity, said the following:
What do you think?