The valuation of Square in the future is possibly not the only factor. Dorsey could be leaving the company, seeking liquidity, relinquishing control of the company or making another investment that has a better expected return. Also, he could be diversifying as he likely still holds a lot of shares. He seems like a smart guy and there are rational reasons an investor would divest a company with a positive forward outlook.

His trades are only a small data point to consider when valuing square. Whether it is worth 14b or not has little to do with him selling 150k shares without additional information.