Personally, I don't care that it isn't Prussia's seiyuu, it doesn't make a difference. The song was written as a duet between between Germany and Prussia, and the people that sing it only enhance it. It could someone entirely different that sung Prussia's part, but the meaning would still be the same, because it's /his part/. It'd still be Prussia's voice coming through. So knowing that it's not his seiyuu doesn't change anything, and I don't think anyone should be throwing that fact around with the intention of raining on someone else's parade that doesn't already know, because IT DOESN'T MATTER. It's still heartbreaking, still beautiful, and still a punch right in the feels to hear Prussia and Germany's words to each other come to life in a beautifully written song.