It appears to be what it is: that the prohibition of federal funding for contracts that limit access to the courts does NOT apply to contracts which are not enforceable in the US.
So, if the employer adds a separate provision to new contracts that stipulates that the contract is not enforceable in the US, (this is the most obvious way that a contract would not be enforceable in the US) then it would seem they have bypassed or sidestepped the prohibition of section (a).
Can employers add a provision such as that?
And if they can, then what is the point of this amendment?!