That price is under optimal conditions. They built in the desert where there are clear skies, lots of sunlight and cheaper land. Plus whatever kind of tax incentives or other subsidies they get in California that conceal part of the cost and may not be available everywhere.
The real question is how far can the price continue to fall before it hits a floor. And what happens to the other generation methods as a result of the competition. If people build a bunch of solar farms and storage and replace natural gas plants with them, then there is less demand for natural gas, so the price falls and the remaining natural gas plants stay competitive due to the reduced fuel cost.
The "solar takes over everything" narrative is possible, but there is no guarantee of it, and it's probably best not to take it as a hard assumption and make no contingencies for if it doesn't actually work out that way.