This is an amusing tweet, let's break it down from a risk analysis point of view.
First, the humor: he's saying "masks don't protect the general public, but they do protect healthcare workers". Um, what?
First, the humor: he's saying "masks don't protect the general public, but they do protect healthcare workers". Um, what?
The reason is risk analysis. Your risk of contracting the #COVID19 is overwhelmingly from touching something with germs on it, then touching your face. Thus, wear a mask, wear a better mask, wear a better mask correctly: it won't change your risk of infection by much.
In particular, surgical masks you see everyone walking around with in China will do little to protect the wearer. That's not their purpose. 
Surgeons wear such masks to protect the patient, so they don't cough into the open wound. Hospitals put these masks on sick patients to protect their doctors and nurses, so the droplets of a coughing patient don't travel far.
China requires people in Wuhan to wear surgical masks in public not to protect the wearer, but to protect everyone else who the person might infect. Especially with this disease, people appear to be infectious without knowing they are sick.
In China, they have vast supplies of surgical masks, because wearing them is a cultural thing. Thus, this outbreak doesn't particularly tax their supplies much more than normal flu season.
In the United States, our supplies are more limited. If there is a large wave of patients due to a pandemic, our hospitals will run out of supplies. They won't have enough to hand out to all the patients. Thus, the Surgeon General is right.
For N95 respirators, it's a similar story. The average person is unlikely to wear them correctly in the first place, and even if they did, probably wouldn't change outcomes much. But hospitals need large quantities for when the big wave of patients come from a pandemic.
This is a good question: don't hospitals have a better supply change than your local Walgreens? The answer is no: everyone feeds from the same supply chain.
Sure, hospitals don't buy from Walgreens, but both Walgreens and the hospital buy from the same distributor. When Walgreens runs out, they run back to the distributor for more, so the distributor runs out, unable to supply the later hospital request.
Hopefully it works the other way around, that distributors can't supply Walgreens with more because hospitals bought up all the masks. And I'm sure, to some extent, this is true. To some extent.
This is a good point. Don't all hospital workers already have N95 masks? I'm assuming the issue isn't current staff, but the anticipated increase in staff if there is a pandemic, getting more people providing care.
