Half right: Cash comp is unlikely to be that high, and that's the GP's point. They could only very optimistically cut payroll to cover $1-2 billion.
Half wrong: Cash comp isn't the whole story, you have to use total comp as equity the acquisition of Twitter also included employee equity plans, so as they vest the company will owe employees a cash value. I'm not sure if Twitter RSUs are dollar denominated (fixed cost, $X/quarter) or share denominated (e.g.: Y shares/quarter). The latter will be more costly because Musk paid a premium.
Half again as wrong: total comp is not the all-in cost of an FTE. Benefits and other associated costs, I don't know enough about payroll to even begin to estimate that.
Also most of those folks on levels also get compensated with stock or equity, so it's more than base. Going private means that goes away and their comp also gets slashed. Is he going to take it back public, what beyond base does twitter offer as comp.