Frequency has to be maintained, and it is trivial to do so when you have excess renewables because inverters are instantly throttle-able. The reason why you’ve never heard about devices failing because frequency is too high is because it is and has always been such a non issue to shutter unneeded generating capacity.

Typically with fossil fuel plants, when the price drops below the cost of fuel for the least efficient plants they drop offline because they are no longer making a profit on fuel and the price holds. Because renewables have upfront cost to build but are free to run on a day to day basis, when there are a lot of renewables the price signal has to drop all the way to nothing before it is no longer profitable to run them.

All this means that all that happened was that for a few hours, solar production was actually enough to satisfy demand for that region. Along term, if low wholesale prices can be counted on midday then people will build industry, storage, or HVDC transfer capacity to take advantage of it.

If these prices are sustained for enough of the day that it is no longer profitable to add more solar farms, then they will stop being built in that area in favor of was to generate power at night such as wind, hydro, and pumped hydro while the panels will instead go to places that still don’t have enough solar to meet demand.

Also as an aside, the wholesale electricity market in north america is by definition about as far from a free market as it is possible for a free market to be without having exact outside price controls. It is a market built solely out of regulation that only exists at all because the government forced it to exist by making it illegal to not use it, either by making contracts off market or by transmission companies in-houseing production, or use it in any way other than as precisely prescribed by the government.

Now we can argue whether or not the wholesale electricity market is well or poorly set up or even if it should exist in the first place, but I don’t think that anyone can argue that it is a free market. At least not without defining the term free market so broad that even most of the markets in the USSR qualify as free markets.

Also, free markets and capitalism are very distinct concepts with no real relation between each other. You might argue that free markets tend to lead towards a capitalist system, but given free markets existed thousands of years before capitalism was invented I don’t think many people would say it was a very strong relationship.