You think they are, but you have to trust that this happens. And even if they are, what happens if the grid is offline and comes online while the inverter is operational, how long does it take, if it does at all, to adjust itself.
If the product doesn't explicitly state this on its packaging there is absolutely a chance for competing sine waves. If this wasn't the case, transfer switches wouldn't have been invented. The original comment I replied to is simply dangerous.