That’s not exactly what they held. They held that the President is immune from prosecution for official acts. Prosecutorial immunity doesn’t mean his conduct is not unlawful; it means he can’t be criminally prosecuted for it. That distinction is important because it doesn’t prevent courts from issuing injunctions. And, as stated by others, the immunity is extended only to official acts. What constitutes an official act isn’t super clear yet, but I think it’s safe to assume that golfing isn’t an official act.
The Roberts court invented the official acts immunity out of whole cloth for the sole purpose of keeping Trump out of jail. They're quite capable of interpreting everything he does as an official act.
