Didn’t he? He may not have needed to know the specifics of why a small group of people was living on Ceti Alpha V, but his knowing that they were there at all would have averted disaster for him, his ship and crew, the science team on Regula I and the training crew aboard the Enterprise.
It’s not as if Terrell was a security risk. He had been part of Operation Vanguard—which, for all we know, may have been why Starfleet trusted him to find a site for Project Genesis—so he clearly could be trusted with classified information. Would it have compromised Starfleet security that much to let a single starship captain know why he couldn’t visit Ceti Alpha V? Marcus might have bitched, but the loss one site would not have set her plans back that much.