Again, he wasn't telling people to do that. he was saying people with a mild case of it may think it's a cold and go about their daily lives.
Yes, that's the problem. It is over twice as contagious as the garden variety flu and because we had trouble getting working test kits in the US, there are probably a lot of folks walking around--and yes, working--who have no idea. Then the bug spreads to someone who is susceptible and gets very very sick.
That's what the problem is. China finally brought this thing under control with draconian quarantine measures. It seems to be the only thing that will work.
I believe Trump's point was that many go to work UNAWARE that they have the virus. He should be encouraging people who show the symptoms to get tested immediately and to stay out of public areas as much as humanly possible until they know for sure they are negative.