there is lots more to it not being an LS engine
It is an LS engine. LS is the generic term used to describe the basic engine family. The Vortec cover on top of the engine in the Monte, the accessory drives and the intake manifold mark it as a truck engine. We are not sure which LS it is yet. 4.8, 5.3, 6.0 - all LS engines. Basic iron block truck engine or aluminum supercharged vette engine - both referred to as LS engines. Same as all small chevrolet V8 engines from 1955 to 199? whenever they stopped making them, are referred to as small block chevy engines no matter the displacement or application.
Vortec is a GM buzzword. Even the mid-90s Chevy pickup engines were touted as Vortec engines and they were old school SBC architecture. The biggest advantage to the old school SBC Vortec engines was much better factory heads. Vortec is used in the same context that GM used to use when they called their 327 engines Turbo Fire V8 - it sounds good in the advertising and promotional information.
LS engines are gaining popularity for the same reason that the small block Chevy eventually gained popularity as one of the most plentiful and popular hot rod engines swaps ever. They are cheap, plentiful, and stupid easy to make good horsepower with. 400 horse is easily within reach with basic bolt ons.