He knows that the only important clue in the room is the pen, but the players do not. Their actions will thus deal with the entirety of the room rather than the metagame thinking that would lead them to the pen. One might decide to check out the broken bookshelf, another might want to check the desk drawers. If they eventually look at the pen and discover its relevance, they will feel that much more of a sense of accomplishment.

This is kind of bad advice if you want the players to engage with the pen. I mean, if it doesn't matter then whatever. But if it's the only thing that matters in the room, spending 10 minutes farting around on other dead ends isn't really a win. Players are already bad at finding clues.

On the other hand, for things that you don't want them to immediately focus on, it makes sense to camouflage them with other descriptions. Like, "The dining room is large enough for a raucous party, but it's silent aside from you all. The stone floor is cracked and uneven- what was once beautiful marble is shattered in places, and stained with what might be blood in others. The dining room table itself is massive- large enough to seat thirty people on a side - and looks to be made of a thick, deeply brown, wood, and seems to be in solid condition. Strangely, it's still set as if for the party. Plates and silverware rest on top of it, as well as a few bottles of wine. {Noble Background Player} you even recognize one bottle as a particularly expensive vintage. The seats, however, are mostly knocked askew or broken. Three are intact, pushed away from the table."

Of course, the table was a giant mimic and the wine bottle was there to trick the player into getting stuck to it. Fun was had by all.