I am going to express a strong disagreement here, not that you could not make a case that she has a high neuro-cognitive capability. But we have learned a lot about high performance over the last decade or so. The short summary is that you have to spend "10,000 hrs" in deliberate exercise of a capability to become and expert at it. She, and I make the same charge against John Roberts, hasn't spent 10 minutes at it, doing the hard work, working through problem by problem and example by example, set exercise by set exercise until you have mastered something.
She demonstrated that with her Barron Trump attack, just like the smartest guy in the room John Roberts did it with his "it's a tax" opinion. They are cheap party tricks not worthy of true masters of their profession who ought to be wiser than the rest of us - not attempt to confound us with transparent trickery, you know - the scantily clad assistant wasn't actually in the box you thrust the sword through. Once you know there is no actual magic, these folks just come across as cheap street barkers promoted to tenure at prestigious institutions.
The problem with the lefties like Karlan is that they let their intellect become subservient to their ideology. So rather than reaching the conclusions required by logic, they decide on the conclusion they want first, and then come up with whatever reasoning they can to justify it. That inevitably leads to them making dumb arguments.
But that's not because they're dumb. It's because advancing leftist ideology is more important to them than making consistent legal arguments.
I knew Karlan when she was a law professor at Virginia. An annoyingly strident leftist hag, but intelligent nonetheless. If you tried to engage her without recognizing that, you'd get stomped. But now...as she's had 30 more years to ingrain that ideology into her legal thinking, the legal analysis just isn't there any more. It's all agenda-driven.
Academics can have great knowledge of a very small field and not know much about the world outside of it and never hear anything that contradicts their opinion. Or they may be highly skilled reasoners and rhetoricians applying their skills in a narrow sphere of acceptable opinions.
Whether that makes them stupid or not is something to be decided on a case-by-case basis. These professors were representative in some ways, but perhaps not in others. Differences of opinion can be attributed to stupidity in some cases, but not in others. If these three professors were stupid, fine, but it can be hard to judge just how intelligent people really are when deep ideological divides get in the way.
As for Roberts, the Supreme Court doesn't like to overturn federal laws if they appear to be acceptable to the public and if the constitution grounds for invalidating them aren't undeniable. So one or more justices will go looking for a way to avoid creating discord and upheaval. You may not approve of the grounds they find, but that doesn't mean they are stupid.