There's no reason to think that the "couple" mentioned on GH didn't come forward. LE doesn't usually release information about potential witnesses after the fact just so the public can know about it. I would assume that if LE are not continuing to issue appeals about it, then they are confident they've talked to the people they need to.
This article is valuable:
State Police asking for public's help in Carroll County ·
In that it was based on ISP press releases and first published when police were asking anyone who was on the trails to come forward, and then was updated with more information over time.
The first appeal by ISP on February 14th was this:
We are asking for anyone to come forward with information or were in the area of the Delphi Historic Trails or Monon High Bridge that might have seen a person or persons, vehicles parked in the area or were taking pictures.
By Wednesday the 15th at 6:50 pm ISP were asking for this:
Indiana State Police have provided a photograph of a subject who was on the Delphi Historic Trails on February 13, 2017 around the time Abigail Williams and Liberty German were walking. The Indiana State Police are asking help from the public to help identify him so he can be contacted regarding what he might have seen.
Also, if you were parked at High Bridge Trail Head on February 13, 2017 between 1:00 p.m. and 5:00 p.m., authorities would like to talk to you. If you can identify the subject or were parked in the lot, please contact the Carroll County Sheriff’s Department or the Delphi Police.
By Sunday, February 19th the ISP press releases no longer ask for people who were on the trails to come forward, but only ask for assistance identifying the man.
My conclusion from this is that LE were able to talk to everyone they felt they needed to....except the suspect.
JMO.