Status: Confirmed, fixed. Redownload (from NeoLemmix website or DropBox) for fixed version. :)
It looks like the 1.36n-B fix for the impossible (http://www.lemmingsforums.net/index.php?topic=2322.0) Frost 12 did not make it into the new level pack system... It was said to be fixed (http://www.lemmingsforums.net/index.php?topic=2340.0) for the next update, but when I checked it with the current 1.38n-B update, the level still has the same skillset and the Lemmings are still walking over the hilltop with no way to stop them.
Hm, not sure how this slipped through. Give me a sec and I'll fix that.
I definitely remember fixing this level, but it seems I must've used the un-fixed version when building the NXP. At any rate; I've re-fixed it, made 100% sure the updated version is solvable, and reuploaded it with the fix.
You only need to redownload the Holiday Lemmings NXP; not NeoLemmix itself.