The presence of a source map doesn't automatically make this open source. It's copyrighted and proprietary unless there's a license that grants additional freedoms.
Open source is different from copyrighted. Or free (as in beer or speech), for that matter. It can be open source, free as in beer, as in speech, and copyrighted, at the same time.
Source available != Open source. The presence of source code doesn't automatically make this "free software" (as in speech, beer, or otherwise.) Copyright is conferred automatically, so a license needs to be granted to make the code Open Source.
