There's kind of a big muddy area between abstract symbols and a full on script, which is specifically symbols representing spoken language. They may have had symbolic meaning without directly standing in for language. For an example in the modern world, a roadsign indicating a turn has symbolic meaning, but that doesn't make it a script. The Nike logo definitely stands for a particular meaning, but it's not part of a script either. It's unknown whether or not the Indus Valley script is a proper script or if it has non-language symbolic meaning.