That is NOT the reason why JM is being sought and not this WG. Wouldn't make a bit of difference if JM came forward. He'd be in as much trouble or more even if he were innocent, and it has nothing to do with color either. You had better believe that LE tracked WG's movements after he was seen on the film and wanted to know what he did afterwards That he is not the last to be with Hannah was also helpful.
JM has the problem that both he and Hannah disappeared off tape after they left at Tempo. So he's the one left holding the proverbial bag and LE can't follow what he did thereafter That's where the problem is. Plus they somehow found out he had a car in the area. So unless JM has an alibi, can prove he left Hannah and the area alone shortly after being with her at Tempo, he has a serious problem. Has nothing to do with cooperation on the part of WG. If WG disappeared into thin air, had a car at the place, LE would be checking him out too. The fact is probably that he left the area and Hannah was then with JM.
The attorney probably told JM that the best thing he could do was not to say anything to LE and lie low.