Do I need a decoupling cap for a line buffer gate?
In theory the chip does not need a decoupling capacitor, what it needs is a power that is within its specifications. To achieve that you can use any means you want, including magic. But the most common way to do that is to have a decoupling capacitor (or mybe more than 1) nearby, and not too much inductance in the feed line and ground. 

So you can say that the power specification in the datasheet indirectly asks for a decoupling capacitor.