It says proprietary because I have not filed in license info and I think this is default value then.
Java runtime is Zulu (https://www.azul.com) so the license will be according to what is license of Zulu.
If check snap content you will see there license file
$ less /snap/java-jre-ondra/current/LICENSE_ZULU
There you will see: The GNU General Public License (GPL)