The most obvious difference between ScummVM and an emulator would be that:
* ScummVM needs much less CPU and system resources than an emulator, as games run directly in ScummVM and not through an emulated platform (which might have a different CPU, different memory management and so on)
* The game can run on many different platforms, thanks to ScummVM's portable platform backend
* Game graphics can be improved with the use of several graphics filters (including super2xsai, supereagle, advmame2x, advmame3x, hq2x, hq3x and more) (yes, I know DosBOX can do this)
* Some bugs which existed in the original game's executable (or specific scripts) may be fixed
* It's possible to re-encode the game's audio files into popular formats, such as MP3, OGG or FLAC so that the game itself takes up much less space
* LucasArts games have a much improved menu and savegame system
* Most game engines allow the player to save much more save games than the original interpreters did
* In some games, ScummVM offers added functionality which did not exist in the original games. For example, it offers full mouse functionality in older Sierra AGI games, which had no mouse support
* It's possible to listen to a supported game's MT-32 music score (if present) without an actual MT-32, via a sophisticated system which emulates it
...and many more. The approach that is taken when implementing a game under ScummVM has some disadvantages:
* Because the logic of each game in ScummVM has been rewritten from scratch, some bugs which were not present with the original game interpreter might exist in ScummVM. For this purpose, there is a bug tracker in ScummVM, where users report such findings to the ScummVM team, which are then usually fixed. Such issues surely exist, as it's impossible to check every single aspect of each game, and a full rewrite of the game engine means that there might be issues which the developer didn't find out originally.
...and perhaps more.
And comparing the ScummVM team to the Scientology church is a bit far of, wouldn't you say? Just because YOU think it's an emulator, that doesn't stop the rest of us from knowing it's not:P See how that works the other way around?
Like you yourself say, an emulator is a program that emulates an os on a different platform to run the desired program through. ScummVM is a program that runs natively on whatever platform you are using.