No, the theory is that a zero day was used on Ulbricht and they knew he was guilty for a long time. Things like seizing the laptop was just theater to construct a parallel trail of evidence for the courts.

I understand that theory, but I don't understand what leaving his laptop unlocked has to do with it. As the FBI already had a warrant to arrest him when they encountered him in the library, they had already made a probable cause showing to a judge by that point. The probable cause showing isn't the same standard as the "beyond a reasonable doubt" needed for a criminal conviction, but clearly the FBI already believed he was guilty before they seized his laptop, whether or not they accurately told the judge about all of the evidence and evidence-gathering methods that led them to that conclusion.

It's unfortunately entirely possible that they didn't tell the judge about all of it, but it's still not as though seizing his laptop was the event that convinced the FBI that he was guilty, or even that they claimed to be particularly unsure about their suspicions before that.