If I were to guess, I'd say that this estimate is including hardware, cloud resources, software licenses, benefits, stock options, and probably more that I can't think of in addition to salary, but I too would like a more detailed explanation on how they came by that number.
Probably also the HR, legal, etc. support staff amortized across headcount.
Fully loaded cost. Typically you estimate that an employee costs 2x their total compensation, across various expenses.
Where is that typically? Most of those are had costs for an employee, not % based. Google has employees from $50K to $1mm+, I very much doubt that all expenses double as income doubles.
How do you know how much your benefits and support staff cost?