If the law says you can only impeach a president in office and Congress insists on an impeachment of Trump. Then they are insisting that Trump is the lawful President and Xiden an ursurper.
Which is what they did. Trump was impeached on January 18,2021.
What is happening now is the Senate is trying the impeachment per Article I, section 4: "The Senate shall have the power to try ALL impeachments".
And, by the way, there is no law which says what you claim.