They did have a valid warrant for arrest, they just didn't have a warrant to search the courtroom but they had a warrant to arrest the guy as soon as he stepped outside.
If I know you're in a building and haver permission to arrest you, it's not "obstructing arrest" if you use the back door. What if your car's parked out back?
To quote the 10 year old who destroyed me in fortnite "Get good."