Psychological experiments and surveys are often done in only one or two days. Why would a wider window of time help when taking a snapshot of a current situation?
Hm, is 258 a small sample size? Let's put in some effort instead of just screaming "bullshit" a bunch. There are approximately 408,000 people in the entire population [0], and plugging everything into a calculator, we get two results: First, they should have tried to poll around 400 people (and perhaps they did!), but more importantly, the 95% confidence interval is only around ±6%.
Let me repeat that more clearly: By typical polling standards (95% confidence interval, random sample from filtered population), we expect that 77-89% of UK software developers are actually burnt out. That is still three out of every four people in the industry!
Not sure what to say. Take a stats class?
[0] https://www.statista.com/statistics/318818/numbers-of-progra...