So... technically that's right: the 917K's flat 12 is a 180-degree V12.
"Flat" doesn't really convey anything specific, but it's usually a byword for a boxer (as in the 911's flat six, which is a boxer). A lot of people insist it's only a boxer, but it isn't necessarily the case - though all boxers are also flat.
In a boxer engine, each piston connects to the crank shaft by a connecting rod on its own journal, leading to slightly offset opposing cylinders and opposing pistons which can act independently depending on crank design (but usually match their stroke so each is top or bottom simultaneously). On V engines each journal carries two con rods, so cylinders can align with each other (but don't have to) and opposing pistons act in opposition - when one is at the top of its stroke, the opposing one is at the bottom. The 917K engine features two con rods per journal, so it's a V12.
From what I know, Porsche chose this route because the boxer eight used in the 908 (by extending the boxer six) was quite unbalanced and it determined that making it 50% longer still might make the issue worse.
The same thing applies to the Ferrari Testarossa, which also has a flat-12 that's a 180-degree V12 and not a boxer.
