Nephi avoids telling a direct untruth when he says he must take the plates to his “brethren” outside the walls. Nephi simply does not correct Zoram’s understandable construing of that statement to mean the “elders of the Jews” with whom Laban had been meeting that evening.
Why did Nephi want Zoram to accompany him? One would think that Nephi’s greatest urge would be to leave immediately and as inconspicuously as possible. Perhaps he required assistance in carrying the plates or thought that he might need Zoram to carry the plates if they were attacked. Apparently Nephi had no concerns that Zoram would become suspicious because Laban had gone out alone to his earlier meeting with the elders. And apparently Zoram accepted without question that Laban would not carry the plates himself. Indeed, it would probably have been suspicious if Nephi-as-Laban had carried them.