Just shut the main breaker off. Always the first step.
“The reason is because linemen or technicians on that line could be electrocuted. This generally requires a disconect switch from the mains.”
For a conventional generator, very true. In this case, I suspect the inverter does that job. All the inverter has to do is shut off output power for a millisecond and then see if output voltage collapses - if there’s no other source (i.e., grid power), then the output voltage will collapse, if there is a source, it stays at 120V. Do that once per second.