No, he didn't sign an EO banning imports on ammo today, he signed an EO that sets the ground work to allow the AG and or ATF to ban imports of ammo or any gun parts "designed for military use".
Here's the ATF bulletin on it (PDF warning)
And here's an article on it.
Basically where we're headed is an eventual ban on the importation of all guns, ammo, magazines, parts, etc. All without Congressional oversight.