This is true only if the company's health insurance is group-rated rather than community-rated. Large employers who self-insure (the employer is actually paying

This is true only if the company's health insurance is group-rated rather than community-rated. Large employers who self-insure (the employer is actually paying the claims; the insurance company is just providing administration and financing) have premiums set based on the cost of caring for their group of employees and will see their costs go up (and their employees' premiums likely go up too) if their employees are collectively sicker. Under the ACA, small businesses all end up lumped together in the same state-wide risk pool, so if my small business hires old or sick people it's very unlikely to affect our premiums. In most places this risk pool is larger and somewhat less morbid than the individual market.