Trevor I believe explains in the post, that being often within M&A there's a window of opportunity to engage a party interested in buying a company or its assets. If he declined, it's possible he would have never received an offer as high as the offer he received and would have to hope the app paid out over the next 5+ years, which was 2x the duration the app had been at its current revenue, which had been consistent for 2.5 years.
It seems the app developer was dying for the thrill of selling his first company. Spanish media is growing, particularly in the US, so there's no reason to think his app sales were about to decline, at least not enough to sell his golden goose for a mere 500k. But congratulations to him on selling his first company.
