AC power needs it's sine wave aligned or it's very bad. If a huge chunk of inverters all start dumping power onto the grid that isn't synced, things break.

You can't just "not sync the grid". I think what you mean is if all the country's inverters that are curtailed stop being curtailed and pull the grid out of frequency because they control so much power. Note that every inverter as well as every traditional power plant and many substations will disconnect themselves long before reaching 59 or 61Hz - the most you achieve like that is a blackout.