And thus it didn't really made sense to me why Epicurus seemed to try and to separate his students from the outside world
Did DeWitt say he did that, or did you pick that up as an artifact of what is generally "out there" on Epicurus? Either way, I don't think he "tried to separate his students from the outside world" at all. I think he taught them to embrace what was good in the world, and fight against what is bad, but in no way did he teach them to pull back into their caves. THAT is the modern or neostoic view that i think is just hogwash -- and says much more about those who say it, than it does about Epicurus, who can't be shown to have said it when the philosophy is viewed in total.