Where have you seen that, what a terrible idea!

In the user manuals for the inverters I’ve looked at installing. Same is true for many battery inverters.

If they need to integrate with a grid supply at all, they must switch at precisely the right frequency. Mains frequency drifts and so that frequency must come from the grid.

Now some will also have a grid isolated mode where they can generate their own frequency when there’s no other option, but that’s not on all models as it’s a feature they don’t need for 99.99% of their life, especially when grid operators generally don’t want people energising the grid from their batteries when the mains is down as it puts workmen at risk. Cables become live at unexpected times. So if you do have an inverter cable of running without mains you also have to have isolation switch so you only energised your own wiring.

An alternative is a separate isolated output that only ever runs on the generated power and not the mains, but that’s a pain for all the rest of the time.