In this particular case, I'm not so sure this is a spoiler. I'm sure everyone expected Luigi to be in the game, and he may very well be there from the outset, thereby making him non-secret.
However, I do feel as if too much of games are shown before release, regardless of who is responsible. I know companies want to show off their games, including features and characters and secrets to discover and all that, but that doesn't mean they have to force feed it to everyone. Really though, the only thing I find necessary is to not put spoilers in headlines. If people read that article, they know what it contains and want to know that information, so there's no need to hide it in spoiler tags in articles. An example is when the cloud suit was announced. The headline should have read something like "New Mario Powerup Revealed" so that anyone who didn't want to know about wouldn't see it, and anyone who did could click the headline. To be fair though, I didn't read the article, so even though I heard about the suit, I don't know how it functions.
Now, I realize every news outlet posts the same story and NWR isn't the only one who does this. That isn't the point. Just because everyone does it, that doesn't mean it isn't spoiler material and annoying. It just means I don't blame NWR. They probably don't want to do things differently just for the sake of it... even though Nintendo does that.

Do you remember the feeling you had when you completed Star Road in Super Mario World and were sent back to the first stage only now all the Koopa shells had been replaced with Mario masks? You're never going to experience a feeling like that ever again.
I remember finding the Star Road, and finding the Special Zone was a surprise because I thought that secret exit was just going to loop around the Star Road like every other secret exit there. An extreme example is from Zelda: Link to the Past, where I had no idea the Dark World existed. I thought the wizard would be the final boss of the game. Experiences like this will never happen again thanks to the Internet.