It appears that the Morgan Stanley Platinum card welcome terms are still different than the other Platinum cards. There is no specific language about not being eligible if you've already received the bonus in the past on this card (or any other Platinum card).
The one statement that could apply to OP's case is the section that begins: "If we in our sole discretion determine that you have engaged in abuse, misuse, or gaming in connection with the welcome offer in any way or that you intend to do so..." and the very vague, "Additional terms and restrictions apply."
Unless you've done something that looks like abuse/gaming, it seems that Morgan Stanley Plats are still set up under completely different rules than the other Platinums. I would look into this further, maybe with the help of your Morgan Stanley rep.
I'm guessing they haven't been as fearful of churning with this card, since the client-advisor relationship probably keeps it from being churned as aggressively.