I think you missed the rest of his actions. The pardon was one part. The other is to review it's classification under the Controlled Substances Act.

You're right, that part is reviewable. (I was only thinking about the pardons).

I don't get it - how does the court get to decide what is a controlled substance and what is not? Shouldn't that be up to some government agency?

It’s up to Congress to make the law. Congress frequently assigns some of that power to agencies, and the courts get to review whether the agencies are following any applicable laws.

It has generally been up the the DEA. When it came time to schedule MDMA it was actually challenged in court by psychologists and psychiatrists along with scientists. The judge recommended a lower scheduling. The DEA said "fuck you" to the judge, petitioners, and the American people and made it schedule 1

https://maps.org/news/bulletin/making-mdma-a-medicine-ii-res...

Just because a pardon can technically be signed quickly and unilaterally doesn't mean that it's pragmatic or wise to do so. As people have pointed out, there's nuances and technicalities, e.g. not applying to undocumented immigrants, that were likely researched and internally debated to limit unforeseen unintended consequences.

And of course, pardons still cost political capital. We all saw how much Biden's people struggled with covid response and getting bills passed -- all of that would be worse if there were fallout from a problematic mass pardon