Just got an email from CBP, see the last line. Applies to land travel only:
Thank you for contacting the U.S. Customs and Border Protection (CBP) Information Center.
In order to limit the further spread of coronavirus, on March 21, 2020, the U.S. reached an agreement with both Canada and Mexico to limit all non-essential travel across land borders. On April 20, 2020, all parties extended these measures for an additional 30 days.
Examples of non-essential travel include:
• tourism
• recreation
• entertainment
If you are healthy and must cross the border for work or other essential purposes, you may continue to do so. Some examples of essential travel purposes are:
• work and study
• critical infrastructure support
• economic services and supply chains
• shopping for essential goods, such as:
o medication
o items necessary for the health and safety of an individual or family
• health, immediate medical care, safety and security
Again, the recent border restrictions affect land travel. They do not affect travelers entering by air, freight rail, or sea.