Because of the realistic nature of Doom 3 (and I saw a preview video. Hold onto your hats.) id decided to leave anything out that would detract from the overall game. First to go? Multiplayer. Because of the nature of multiplayer games, where individuals have to be able to traverse a map in any direction, often times level designers make several changes to enhance the multiplayer experience. With Doom 3, because of the workflow, an artist would create a room and it would take a week and a half to program. Any changes to the room would require reworking so id chose the economical route.
Considering id has maintained that Doom 3 will have a basic deathmatch mode at the very least, and no one else has seen fit to report such a minor detail as Doom 3 being SP-only, I would take this article with a fairly hefty boulder of salt.