They do account for electricity costs in the "gas savings":

"The average person drives between 10,000 and 15,000 miles and spends between $1,000 and $1,500 on gasoline per year. In comparison, the cost of electricity to power Model 3 over the same distance is up to three times lower. Over the six year average length of car ownership, that's between $4,300 and $6,400 in gasoline savings.

We've assumed a fuel economy of 28 miles per gallon for a comparable gasoline powered sedan, for example, the 2017 BMW 3 series. We've also assumed the national average of $0.13 per kilowatt-hour for electricity and $2.85 per gallon for premium gasoline over the next six years."