I can only guess, but what my books say is this:
Ken Schramm's book mentions:
P64 Dessert Mead 1.020+
P19 Sack mead, or sack, is a strong, sweet mead
Another mead book I have doesn't say anything specific, but skits around the issue by saying that for sack meads you need a Starting SG of over 1.15, which hints at high ABV.
It leaves me with the impression that all sack meads are dessert meads, but not all desser meads are sack meads. I could be wrong, but thats my impression. So off the top of my head, I would say an 18% ABV mead that is very sweet would qualify as sack, but it would be both technically.
just my thoughts, rambling as usual,
hedgehog