Fortunately for me I'm happy with my own definition of "success".
I wouldn't say it's that they don't get stressed, but that they are able to quickly compartmentalize it and move on without dwelling on it.
On the "are they just great at hiding" question, at least in my experience this isn't really possible. Personally, I have severe negative physical effects in response to long term, chronic stress. I sleep poorly, get severe intestinal issues, etc. At some point it becomes pretty impossible for me to just "hide it" - I'd LOVE to be able to quiet my body's physiological responses, but I'm just not capable. In seeing how some of my colleagues dealt with the same stressors, I can assure you they weren't just "hiding it" - if they were experiencing the same physical effects I was, at some point it would have been impossible to hide.
In any case I think that it would be safe to say that sociopaths have at least the embedded advantage of being "quicker", as they most probably ponder less than other people about in/direct consequences of their actions.
Without getting too political, take Donald Trump, who in my mind kind of exemplifies this definition of success. Whatever your thoughts about Trump, he's definitely not lazy, and his early morning tweet storms seem to signify that he gets by on little to no sleep. Of course, he may cause WWIII in the process.
being swiss, if he gets little or no sleep (because or routine work or anything which he triggered himself) I would judge that as mismanagement :)
To be fair, in my mind this is the one most common trait I find in successful entrepreneurs and leaders. I've seen leaders who I didn't think were particularly bright, or extroverted, or empathic, but man, did they not get stressed out by obstacles that came in their way. They either sidestepped them or overcame them, but they did not dwell on them and let those obstacles lead to self doubt.
This is one reason why I think sociopaths are over-represented in leadership positions. They are not burdened by a conscience that can lead to self doubt in times of stress. Of course, it's possible to have this "does not get discouraged by big obstacles" attitude and not be a sociopath, but it is certainly an area where sociopaths have a natural advantage.