Why are lubavichers always trying to push their own agenda as if it’s standard and accepted?
There is no such thing, and there never has been, as “Shnas Hakhel” in reference to anything other than the fact that it is an easy way to refer to the year following Shmittah, due, of course, to Hakhel taking place then on Sukkos.
A quick search of the latest version of the Responsa database shows that the expression Shnas Hakhel appears in the entire database only four times, with the earliest being from the Chida. And even that is only in reference to the event itself taking place that year.
There is also no reference to the importance of a gathering on Sukkos.
If the Lubavicher rebbe wanted to have his chassidim do something in memory of Hakhel, that’s fine with me, but it’s just another case of lubavich inventing something that the entire world is expected to agree with.