In the intern's place, I probably would not have bragged about copying that company's idea. Then again, as the company, if I felt a former intern could threate

In the intern's place, I probably would not have bragged about copying that company's idea. Then again, as the company, if I felt a former intern could threaten my business like that, I'd ask serious questions about my value proposition and moat (which could be premium features, sales, or an attractive/viral free tier - not too familiar with what they do). On a personal level, I can see how it could feel unfair, but the idea does not seem that original.