Will somebody clear this up for me?

Today TFG’s lawyers filed a response to the DOJ’s filing in the matter of TFG vs. The United States of America. In that response they injected a new and intriguing argument with regard to the Presidential Records Act.

They claim that in a clause from the law, there is a flaw in the law’s wording.

Quoting: “…the Archivist of the United States shall assume responsibility for the custody, control. and preservation of and access to. the Presidential records of that President”.
(emphasis is mine).

Their argument? The law fails to use the word “must” instead of “shall” because they mean different things, even in the context of the clause’s wording.

I’m no legal eagle, but I see both words as directives. And I’ve been told by legalese people that should know that there is no difference between “must” and “shall”. Am I wrong?

Anyone beg to differ?