Correct, I'm not having a sleeve. Friends of mine had the sleeve version and gave up I on the sleeve, putting the mat inside the hammock, just as I'm doing it without the sleeve. Can't call out any drawbacks.
If you wanna give it another shot, the sts etherlite insulated is especially fat. Though I had a friend freezing his butt off on it in a hammock in the ~5°C temps. So not only the indicated weight, but also the R value might be quite a bit off on that pad.