The decision addressed that. They were asked not to take it because there was no constitutional issue due to Congress not being in session, but the Court thought it mattered that any criminal proceeding arising from the arrest could keep him from a session.
In any case, McKinney would be guilty even under the defendant's argument, because McKinney did indeed commit a crime that is directly under breech of the peace ("offenses involving violence or public disturbance").
"Since from the foregoing it follows that the term 'treason, felony, and breach of the peace,' as used in the constitutional provision relied upon, excepts from the operation of the privilege all criminal offenses, the conclusion results that the claim of privilege of exemption from arrest and sentence was without merit"
Even in session she's not exempt.
Members of Congress are exempt from committing such a crime within House Office Buildings when in the lawful discharge of official duties.