It is not clear that any other customer's metadata was "uncovered" by the police request. According to the ACLU report on US v Carpenter, the request was for records relating to the accused, Timothy Carpenter and Timothy Sanders. It does not seem to have been a "fishing exercise."
It would be reasonable in circumstances similar to those in Carpenter for police to obtain a warrant. There was an ongoing investigation, and they clearly suspected Carpenter, Sanders, or both, and might well have been able to get a warrant based on probable cause without a great deal of difficulty. Superficially, though, this case also seems similar to Smith v Maryland, in which the Supreme Court found a warrant for metadata unnecesary, but also somewhat dissimilar in that the location data sought in Carpenter was a given in Smith, and did not exist in the same sense. Metadata now is much more extensive and problematic than it was in 1976, and a different answer in this case will not necessarily overturn Smith.
If Carpenter wins, he and maybe his accomplice are likely to get no better than a retrial without the phone company location data, and may yet be convicted on the other evidence available to the prosecutor.