I'd be interested to know where the numbers for the general election came from.

As best as I can tell based on what he says on the site and how he does baseball stuff: I bet he takes as many polls as he can get ahold of, assigns weights (and possibly adjusted MoEs) to them based on the age of the poll and possibly past performance of the pollster, and then runs a simulated election based on the aggregate of that poll data. Repeat the simulation a ten thousand times, and then report the results.