Correct, Schiff is not a law enforcement officer. But the HPSCI which he chairs definitely has the subpoena power to obtain such records for an investigation like this.
Giuliani, Nunes and '-1': A Look At What The Impeachment Report Phone Records Mean
The phone records were presumably obtained through a subpoena issued by the Intelligence Committee, but the report never specifies how the logs were secured.
A spokesman for AT&T, Jim Greer, would not comment on the records, only saying the company complies with information requests from all law enforcement agencies.
"In all cases, we ensure that requests for assistance are valid and that we act in compliance with the law," Greer said.