As you said, this would require an additional action.
And to me it looks like it can’t be done by EO and would have to be a legislation. And during trump very few legislations got passed even when they had a majority.
The restrictions on sale through Mexico can be implemented when it actually is a problem.
I found an article where they announced a factory in Mexico, but it first have to be built. The news also appears to be before the sanctions were announced. So if that factory was meant for US market this could change their plans, as adding 100% sanctions signals determination to plugin other loopholes.