Almost certainly not a legal download. Or at least not a legal upload.
The same uploader seems to have also uploaded Toonstruck, Torin's Passage and other old P&C games, I can't believe those are all legal as all of them are being sold at the moment.
The only way I can see those being somewhat legal is if they come from some collection which has unclear or ambiguous wording about the terms of use.
_ChaosFox_: Second: No, it doesn't have the same codebase. The special edition was remade more or less from the ground up - the "classic" mode in the background was essentially remade based on the assets from the original game.
Are you sure about this?
I know that's how Double Fine remakes were made, but I seem to recall a story that Monkey Island special editions are, in fact, based on the old code and there's a special wrapper which interprets the old code to newer additions, such as graphics and sounds.
Then again I really have no absolute knowledge about this, but there's certainly at least a convincing rumor about those SE's being based on the old code.