From the article:

And when there’s a power outage, the PowerStream will turn off automatically to ensure there’s no electricity in the wires in order to protect line workers from shock. The PowerStream will only turn back on when the grid power returns.

Transfer switch is the proper tool for this and is a fairly simple install.

Yeah my parents house had an rv/generator hookup and it had a huge bar across both the breakers so power could only flow in one direction. If you hooked up a generator it would cut the house off from the mains.

Your grandfather's extra outlet for the alternate feed was the other half of a switch that flipped over when the mains power died. It shuts off the power connection to the house by flipping over and ensures no power goes back over the line, among other things. We have these - albeit the size of a washing machine - in really big datacenters.

Depends on electrical code which depends on, most of all, your standard plugs. In Germany Schuko is deemed non-optimal, but acceptable, for up to 800W.

...no issues regarding exposed prongs, if the inverter doesn't see AC to sync to it doesn't output anything. It's not a dumb spinny magnet generator we're talking about here.

Most people don't have an outlet on their balcony, though, and weather-proofing the thing is an issue in any case so while you're at it you can just as well put in a proper Wieland outlet. 20 bucks or so, the expensive part will be the electrician not the outlet.

But now some genius doesn’t read the fine print and hooks up 2 or 3 on the same circuit. There is no electrician that tells him that’s dangerous because it’s all self installed and he doesn’t know any better. And all of a sudden you are up to 26.5A and you got glowing, smoking wires in your walls…

Ok sure, that makes sense. This might actually be an issue.

I know enphase micro inverters have "anti-islamding" tech that disables output when they lose grid connection. I would expect any reputable manufacturer to also have the same tech.

I don't expect that for backup generators, but the proper way for them is via a transfer switch. You can wire in a properly made cable instead of a suicide cable. The transfer switch would prevent the inlet connector to the house from ever being live. (And since it is a proper cord from the generator, there would be no exposed ends coming from it.)