As a non-American, this confused me: "$100k as a freelancer — in other words, pre-tax — amounts to around 60–70k as a staffer (...) since we have to pay tax on our earnings." Does that mean staff job salaries are listed as an amount AFTER tax is taken off or how does this work?

Here in the UK, if a staff job is listed as £35k/year, that is the amount before tax. You still need to earn more as a freelancer to get the equivalent amount, but that difference is mostly in paid vacation days, pension contributions (usually employers would pay a fraction of those), expenses like conference travel or equipment, and a different rate for NHS contributions (health insurance).