The only taxes I could even think they would be privy to would be taxes for tax years WHILE DJT has been President, and even then, I think they would need probable cause. Tax years prior to Presidency are frankly no ones business, IMVHO.
Congress has two paths it can take. Repeal the privacy law and make all tax return public records, or come up with a legislative purpose that is not pretextual, as this demand obviously is.