The link provided for the KML on github.com is being served with a MIME type of "text/plain", it should be "application/vnd.google-earth.kml+xml"
3 KML MIME Types
When responding to a request from Google Earth (or any Earth browser), a KML server must follow a certain set of rules so that Google Earth can correctly interpret its responses.
Upon success, the server must return a response code of HTTP 200 and set the response's content-type to a suitable MIME type, as described here.
Google Earth reads KML and KMZ files. The MIME type for KML files is
application/vnd.google-earth.kml+xml
The MIME type for KMZ files is
application/vnd.google-earth.kmz
validator results:
Recommendations
This feed is valid, but interoperability with the widest range of feed readers could be improved by implementing the following recommendations.
Feeds should not be served with the "text/plain" media type
If I copy your KML to my server (which serves KML with the correct MIME type), it works
BTW - If I point the validator to the link in the code in your question, it doesn't work at all, it isn't a KML file, but if I point the KmlLayer URL to that, it also works