It's not only salaries:
about half of Signal’s overall operating budget goes towards recruiting, compensating, and retaining the people who build and care for Signal. When benefits, HR services, taxes, recruiting, and salaries are included, this translates to around $19 million dollars per year.
Yes, I agree it's a lot.
I think that with "recruiting" and "HR services" they mean outsourced services, so maybe not all of it goes directly to the employees.