actually it is you who is not understanding. so let me help a Brit out.
the US has 3 Branches of Government & key to those 3 branches is the separation of powers.
the US Government Accountability Office (US GAO) is a branch of the Legislative. the same branch as the Congress. The US Government Office of Accounting was created in 1921 & in 2004 changed Accounting to Accountability, (i am sure nothing else changes as a result *snark)
yes the legislative does make laws & if those laws are sound & do not harm the separation of powers, or extend the power of Congress (the legislative) to the detriment of the other 2 branches, the Executive (the President) & the Judiciary (Supreme Court & lesser courts) they tend to go on year after year unchallenged.
The US GAO ruling is non binding. they aren't the Judiciary. to get that ruling to be binding, they need to involve the Judiciary (civil suit.)
so whilst the timing of this ruling, 2 weeks ago, is no surprise. it means next to nothing without trial. just ask Barack Obama: Seven Times the GAO Found the Obama Administration Violated Federal Law. and no i am not saying this excuses anything. just pointing out these rulings aren't exactly as incriminating as you seem to think.
so if the Judiciary is involved, you need to ask... how was Congress harmed by the timely Deferrals of Ukrainian Aid by the US Office of Management & Budget (US OMB) & President Trump?
you should also consider the US OMB & the President part of the Executive branch with its own "Separate Powers"
additionally how historically has the Judiciary reacted when reviewing spats between the Legislative and Executive Branches?
perhaps drop that British mental framework and understand this is the US with a whole different rule set.
OK so tell me, have you read section: §686. Reports by Comptroller General of the Impound Control Act?
it is a pretty important provision should a President fail to transmit a special message.
if you have read it do you understand it?
and if yes, can you tell me what went wrong?
the January 16, 2020 GOA decision really isn't all that surprising & i am really looking forward to November 2020!