They joined Luther's movement after the fact, so that makes them apostates. Had any of the bishops actually joined him before he was excommunicated, the issue would have been different.
Lutheranism was ushered into Germany without a single bishop behind Luther. As a priest without a bishop, he had no authority and was a rebel, renegade, priest.
"They joined Luther's movement after the fact, so that makes them apostates."
From a Roman point of view, I should think the same reasoning would apply to us, Kosta.
Inasmuch as they adhere to Luther's philosophies of the four Solas, they are apostates, regardless of the provenance of their episcopacies. Apostolic succession requires two things: valid Holy Orders and confession of faith of the apostles. The former may be intact here and there, the latter is grossly lacking all across the board.
The Anglican succession suffered a similar fate later: at some point, the confession deviated from the deposit of faith so sharply that Leo XIII pronounced it breached.