I suspect one reason the Supreme Court did not cite anything for the first sentence in this paragraph of Carpenter is that it was invented by Carpenter's brief. Which makes the 2nd sentence kind of amusing: As Chief knows, Riley's discussion has nothing to do w/ what is a search. 
2/ The Court has in the past referred to "diminished" expectations of privacy as a reason why a search might be reasonable without a warrant. In the cost/benefit of reasonableness, the thinking runs, a strong govt need for search outweighs diminished expectations of privacy.
3/ By claiming (wrongly) that the 3PD was about "diminished" expectations of privacy -- rather that the elimination of them -- Carpenter moves the concept over to a different doctrinal box of what is a search and then requires a warrant even to the "diminished" expectation.
4/ Probably reads well to someone not familiar with the cases, but it's kind of strange twist in the doctrine. On the other hand, the old cases gave Carpenter relatively little to go on, so no surprise they had to get creative and repurpose old doctrines to get there.
5/ And to be fair, some lower court cases did talk about the transmission of contents of communications such as e-mails "diminishing" expectations of privacy in them. It didn't make much sense when they used that language, but some cases did use it. SCOTUS didn't, though.
