Stan - you are correct.
G - my other PS3 steamer that does coast is a 30-1570-1. I also have a PS3 30-20053-1 (Dash 8 Diesel) that does coast just fine. I did have, but sold last year to fund another purchase was an 30-20211-1 ES44 Diesel. The ES44 also coasted just fine.
I do not have any other PS3 engines. Per MTH/Andy, two motored diesels and steam (articulated) should work just fine. I certainly hope my long awaited 44 ton switcher does not have a no coasting issue. Andy said I should not. The loco that started this "no coasting" issue was an 30-1651-1. I did sell it to again fund another purchase.