Even if the wise one is under torture - stretched on the rack, he is experiencing eudaimonia."
I recall in my college philosophy class that the professor generally translated that as having a "good spirit."
It seems like the usage in English of "happy" over time has changed, but regardless of that there's no doubt that a normal person today hearing the word "happy" is going to understand at first glance something much different than what was being talked about by Epicurus and the others as well.
Having a good spirit would also appear to be something of an idiom even then - certainly Epicurus did not consider there to be anything supernatural involved in it, regardless of what Socrates might have implied about having a "daemon" talking to him.
What does that lead to? Probably to the continuous need for up-front and early discussion of what "happiness" really means when describing Epicurean philosophy, just like explanations are needed for "pleasure" and "gods" and "virtue."
Epicurus has probably given us the best example possible by writing that letter on his last day. That makes it unmistakeable that happiness does not require total absence of and separation from pain.