This isn't about Congress at all, or about any procedures it might have established regarding the President and classified information.
The question is, how does anyone know whether or not Trump actually declassified documents while President. What is the evidence that he did that, by any method at all?
If Trump refuses to comment, then the answer may be, "nobody knows". That may be the answer for all time.
Biden didn't and probably couldn't make a claim for having re-classified the documents, because presumably nobody knows which documents are involved. A judge issued a warrant against a former President based on assertions that Trump had classified documents in his possession.
If, as you and others have suggested might have happened, that assertion is a lie. The judge was supposed to weigh the evidence in the application for a warrant and determine that a crime probably had been committed and that evidence substantiating that was probably at Trump's home.
I totally disagree. The persecutors of Trump are proceeding completely on the basis that Presidents and former Presidents are subject to legislation involving classification and declassification of documents. Without that legislation the DOJ and the rest of the conspirators would simply have left Donald alone.
Donald Trump has far, far more national secrets inside his brain than he probably ever had inside his house. That is a simple fact of life in our political system. Trump is being treated differently because he is a threat to those who are attacking him.