Is that a house rule, and if not whats the source?
It's a fallback to the fact that the DM can do anything. It's not stated in the MM that they have extra powers outside of combat, it's merely the mantra that's been adopted in response to the fact that many of the more interesting monsters have had a lot of detail removed from their write ups in the new edition.
So basically people complained that monsters in 4E are cardboard cutouts, and the response was "well it's up to the DM to do all the work of actually giving them abilities." It's not explicity stated, it's just something that a DM will need to do because there are gapeing holes in the material.
For instance Vecna doesn't have access to wizard spells in or out of combat, which, given his background, is ridiculous. Instead he's nowhere near as good as an archmage should be, heck, an apprentice casts magic missle better, he just has some weird abilities and a sack of HPs far greater than a player's instead.
If, as a DM, you want a Vecna that doesn't feel like a cardboard cutout, you have a lot of work ahead of you. "For a being of godly intelligence and power, Vecna’s tactics are fairly simple."
