While the fully burdened cost of an engineer being double his salary sounds suspicious, this is indeed broadly the case. It has been (sometimes significantly) more than double in the case in every US employer where I worked and where I saw both numbers. In one case it was a hair under 3x.

My experience was not with pure software houses; we had some labs, measurement and RF equipment, but even without the hardware component the offices, insurance, admin expenses, HR, janitors, conference travel and so on would easily bump the total employee cost to double the salary. My 2c.