“The discovery of that warrant broke the causal chain between the unconstitutional stop and the discovery of evidence by compelling Officer Douglas Fackrell to arrest Strieff,” Thomas wrote. “And, it is especially significant that there is no evidence that Officer Fackrell’s illegal stop reflected flagrantly unlawful police misconduct.”
The Fourth Amendment protects against UNREASONABLE searches and seizures. Thomas is saying that regardless of the unreasonable stop, the police had a duty to arrest the guy based on an outstanding warrant which adds up to a reasonable search in the arrest. That seems like a reasonable conclusion. Sometimes it helps who is for and who is against. Thomas usually hits the nail on the head and the Ginsburg, Sotomayor, and Kagan dissent gives the decision more credibility IMO.
It's not reasonable.
Thomas's whole reasoning is like saying: well the prison guard, disoriented and confused, stabbed an unknown inmate at midnight... but it's ok because that particular was supposed to be executed in a week.
IOW, just because there happened to be some actual reason to arrest the guy (the outstanding warrant), the police didn't know it at the time and it's essentially just random chance that someone got caught. (But then again, with the TSA we know what the government's stance on the 4th amendment is, right?)