Here is an interesting factoid for you:
Just like Pope Frankie, Luther is not infallible.
Plus Luther is not our “Pope.”
I think the difference here is that Luther not only did not claim to be infallible, he did not claim to know for certain how Mary travelled to her present existence, whether it be by assumption or by physical death. I will leave it to the other Protestants to argue that it could not have happened, accept Luther's suggestion that the issue is adiaphora, and leave it at that. I don't have to either accept or reject the Assumption to stand with Catholics in the propagation of the Gospel, or in all those doctrines of which the Catholic and Lutheran churches are in agreement, which include all the doctrines necessary for salvation.
Protestants love the “protestant” things about Luther, but he can just keep all those “Catholic things” he believed in till the day he died.