question

From the docs:

ESS is not supported

The Multi RS does not yet support grid codes, and therefore does not yet support ESS installations. It is not yet known if the current hardware revision will be able to support grid codes, or ESS, via a firmware update in the future.

For about one month I've been playing with my own multi RS. This is what I have observed using a shelly power meter on the input:

there is a grid to output AC switch in the multi RS. When the switch is open, there is absolutly no feed in possible. Power is 0W, fair enough.

When the switch is closed and there is

1. an AC load present, then a small portion of the load is supplied from the grid (for me 50W to 100W) and the rest is provided by the inverter. I guess this gives margin to avoid feeding in.
2. no AC load present, the standby losses of 15W are drawn from the grid even if the solar charger is active and the battery is full.

if the switch is closed but the battery is empty then the inverter complety stop and the full power is suddently taken from the grid. The solar charger is then fully redirected to the battery and won't be used for your AC loads. Mind that this can trigger 16A circuit breakers easily as the inverter of the multi RS is able to output 25A at full power.

The switch can be automatically open if the load is below a programmable value or if the battery SOC is high enough. This way you can get rid of the last bit of power drawn from the grid when there is no need for grid support.

I do agree that the documentation is kind of obsure when dealing with ESS on the multi RS, especially with the side effects of NOT supporting it. As you said, the demo on victron connect is not helping and hint that not supporting ESS still mean feeding into the grid. For all I can tell uncontrolled feeding in also belong to the category of "ESS is not supported".

However, from my experience not supporting ESS mean in victron language "not feeding in".