To me scrappy is the use of different fabrics that you have in small quantities. In other words, it is not coordinated yardage.
However, scrappy does not necessarily have to be 'leftovers' from other projects. The colors can be coordinated or not. That choice is up to the maker. If you insist on having all different colors in a scrappy quilt then that's up to the maker. But if that's the case, then there are quilts that could not be made for several years. There are several quilt patterns that traditionally are made in one color scheme (i.e. Ocean Waves aka Village Green are traditionally made either blue & white/muslin [signifying the blue of the ocean] or green and white/muslin [signifying the color of grass]. Granted you could make them out of various colors but the maker may want the color scheme look.
Being made strictly from leftovers is the traditional view. The word scrappy coming from the word scrap. If you really want to be a purist, then those scraps must be leftover from clothing construction or old clothing because that's where the scraps originally came from. Not from other quilt projects.
In today's society, there's also the situation where someone doesn't have the room or the finances to buy yardage (especially at today's prices) and then don't have leftover scraps to use. So they chose to buy fat quarters instead. There's nothing wrong with that. The quilts made from the pieces from those fat quarters are not any less scrappy than those made from leftovers.
That's the best thing about quilting as an art form. One scrappy method is not any better than the other scrappy method.