I wouldn't jump to dark conclusions about this decision, and certainly don't think it's because 'they' know more than they are letting on. 'They' know it's a communicable disease with an incubation period of about 10 days. Quarantining is a time-honoured, effective method of dealing with such issues. I'd be more concerned about people not following a voluntary quarantine than I would be about having an involuntary quarantine ordered.
I'd be more inclined to view the order as putting a key transmission prevention requirement in place to make sure the health system can effectively block any spread of SARS.