There's no mention that the startup "refused" the buyout, and it was unclear why the negotiations fell apart.
The actual article is simply titled "Tesla offered $325 million for Salton Sea startup".
And here's a quote from the article giving a different picture : "The person with knowledge of the negotiations told a slightly different story, saying Musk shut down all discussions after seeing the $2.5 billion valuation from Jefferies. According to that source, Musk abruptly ended all negotiations after reviewing the valuation, never giving Simbol the chance to present a counteroffer to his proposed $325 million purchase price."