I think you're misinterpreting the announcement which says:
Supported versions of Elasticsearch (6.8.9+, 7.8+) used with recent versions of the JDK (JDK9+) are not susceptible to either remote code execution or information leakage.
It doesn't say that 7.7 is affected, just that it's not a supported version (i.e. it's past EOL) so it's out of scope.
2021-12-16 edit to add: "out of scope" meaning "out of the scope of this particular sentence". There are other parts of the announcement that relate to EOL versions.