Although Remembrance of The Daleks was the beginning of the 25th Series, wasn't the actual 25th Anniversary Episode The Silver Nemesis?
That's correct. The first episode of "Silver Nemesis" aired on November 23, 1988. "Remembrance" had anniversary elements, but "Nemesis" was the official anniversary serial. But since "Remembrance" was awesome and "Nemesis" was lame, fandom has embraced "Remembrance" as the "real" anniversary serial, and thus we sometimes forget that it wasn't.