No you are wrong. The court has ruled on whether he committed "obstruction of congress" simply by taking this up. If they rule in his favour (which is the expected outcome) then he has, as a separate branch of power the right to withhold witnesses for executive privilege which I believe he will win on from years of precedent based on the separation of powers.
If he loses then and only then is he compelled to send the witnesses that the SCOTUS deem are not covered by executive privilege.
By taking up the case they invalidated straight away the 2nd article of impeachment by displaying to everyone that he has a sound argument that they will deliberate on. The Democrats know this hence they expedited the process to get out in front of the courts as there's next to no chance they win and as you see above even if they do win he hasn't "obstructed congress" as it's his\the executives right to test the validity of a subpoena before adhering to it.
