My one trachy , which is protected only with mulch, and not very high mulch at that, I figure is as good as toast. It survived both vortex years (2014/2015) nearly defoliating both times, in a NE facing yard. (lowest temp was around 2F) Its nice that it had two mild winters after that, and its recovered completely, but I think this is IT for it.
I would note that in DC, 2014/2015 killed at least half the known trachycarpus palms, WITHOUT EVER GETTING CLOSE TO ZERO. It was the durations below .. some temperature, Maybre Freezing, maybe 20 degrees F, plus maybe wind or lack of hardening/etc. So yes, it doesn't have to go to zero. I will tell you that a brazoria I had up against the wall facing east did much better with only about 30% leaf damage.. (and only mulch). I think the hardiest sabals are more used to East Coast continental blasts than Himalayan/etc trachines are.