From the oral arguments it was immediately obvious that Alito and Thomas had already decided their opinion --- as had the other judges, frankly. They were very skeptical of the ByteDance/petitioner's argument. The Act at issue was written in a very specific way to neuter a lot of their points. Elizabeth Prelogar, representing the US Government, is also an extremely good SC lawyer in oral arguments. A Per Curiam decision is not surprising at all, most people who follow the court were expecting it.

I think it is often the case that the justices' opinions are already established, based on their lines of questioning.

In the way that Gorsuch wrote a separate concurrence, I expected Alito or Thomas to want to broadcast a particular message to their audience.