Snoke is Snoke..not Plageuis not Tarkin...just Snoke, and The reason why I think this is simple. For example our President is called "President" long after he has left office. If Snoke was Plageuis why would Snoke want to be called anything but Plageuis. Why not Supreme Leader Plagueis? Obviously S.L.S had been around for a while. I'm sure if S.L.S went by anything other than S.L.S.. Luke, Leia, and Han would've known and not called him Snoke. I'm sure that if Plageuis came back from the Depths of the Darkside Luke would've felt it and wouldn't have allowed Ben Solo anywhere near Snoke.