Earlier quoted context omitted.
Congress hasn't given blanket permission. The law says that it's illegal for certain government officials to accept gifts etc. It doesn't say anything about whether it's legal for the president to accept them; it merely says that the president is not within the scope of that particular law.
You don't have to have a law for something to be legal. All actions are legal unless stated otherwise.
Constitution has a clause that prohibits it for any public official including the president (assuming for the moment that it is the meaning of the emoluments clause), unless Congress specifically authorizes it.
Congress additionally passed a law - 18 U.S. Code § 202 and § 208 - that regulates conflicts of interest among government officials. Its domain intersects with the Emoluments Clause somewhat, but it is not an implementation of the Emoluments Clause. It says that certain actions by people other than the president are illegal. Insofar as it intersects with Emoluments Clause, it is at best redundant (although even then it defines clear penalties, so not really).
But there's nothing in it authorizing the president to receive emoluments. § 202 does contain language excluding certain people from the categories of "government officer" and "government employee", which are the target of § 208; but this simply means that § 208, and prohibitions in it specifically, do not apply to those people. In other words, the law doesn't say anything about them. It most certainly doesn't contain any language explicitly authorizing emoluments. Here, see for yourself:
https://www.law.cornell.edu/uscode/text/18/208 https://www.law.cornell.edu/uscode/text/18/202
If we look at other past cases where the Emoluments Clause had to be dealt with, Congress specifically authorized such things, with clear verbiage to that effect. For example, here's the Foreign Gifts and Decorations Act:
https://www.law.cornell.edu/uscode/text/5/7342
Note the language: "The Congress consents to ..."
There's nothing even remotely similar in the law we are discussing.
The only real question for the courts here is whether receiving payment for goods and services from a foreign government qualifies as emolument or not. The originalist interpretation is in favor of that - this is one of those clauses in the Constitution where the Founders wrote a great deal about why it was there, and it's clear that their concerns were broad rather than narrow (preventing "foreign corruption" of all kinds).