To put more concisely, she won because there's an expected temperature for coffee, they served it significantly higher than that without notice, making it difficult to properly assess the risk of the situation.
Common sense only applies when something is common knowledge. Making their product not conform to common expectations in a way that would be non-obvious, with no warning about the difference, is dangerous, and where they got in trouble. If it were standard temperature coffee, or just an anomaly that heated it more, they'd likely have won instead.