They are not exempt from the ACA. No other employer-paid healthcare has been cancelled by the ACA and requiring them to purchase from the marketplace. Because these staffers are employed by the government...which obviously employs more than 50 people, they have employer provided insurance. JUST LIKE EVERY ONE IN THE US! The marketplaces were established to provide a reasonable and affordable choice to everyone who was NOT covered by employer-provided insurance.
Forcing the congressional staffers off the Federal Emplouees Health Benefit Program is equivalent to cutting the salaries these individuals are being paid.