The engineer's statement linked Syed to the location of the victim's body. According to The New York Times, though, the engineer wasn't shown "a crucial disclaimer about cell tower data that would have affected his testimony." Brown didn't detail what that disclaimer and cell tower issue were, but the judge's retrial order said there was a "failure to cross-examine the State's cell tower expert about the reliability of cell tower location evidence." It was only one of the reasons why Syed is getting a second chance to defend himself, though.
The other reason is that the defense team secured the help of an alibi witness who wasn't called in the first time. A woman named Asia McClain testified that she was chatting with Syed in a library while the crime was being committed. If her name sounds familiar, it's because she was also a prominent character in the first season of Serial, as host Sarah Koenig talked about Syed's alibi.