All of the shirts or baselayer tops are long sleeved. In hot weather, long sleeves keep me cooler by not allowing the sun's radiation to have direct contact with skin. Short sleeves actually make one hotter as the sun's energy directly heats the skin and works that heat through the upper layers.
The long sleeves will dramatically increase thermal warmth in cool or cold weather. Since I have two long sleeve shirts (one in the pack and the one I wear), I can add the second which really ramps up the insulating factor. Add a vest and the comfort range is pretty significant. Add a wind shell or rain jacket on top of that, and it is surprising just how warm you will stay at low temperatures.
No, I don't use Zpacks packs, but I have other pieces of gear from them. I was hired by them to do a gear test of their ArcFlex, and While it was OK, I never could get used to it. My current pack is the Gossamer Gear Mariposa. I've used it on part of my PCT thru-hike, on my Colorado Trail thru-hike, and on last fall's Camino. It will come with me again during this September/October Camino. And unlike the ArcFlex, the Mariposa meets all standards for use as a carry-on for flying.