I fondled one. It is a bit odd. Seems to mimic some athletic apparel sports gear my kids wear. Sleeveless hoodies, short sleeve jackets and what not.
My quick take = Short sleeves. No liner. Super light, super specialized piece of kit for someone looking for the barest minimum weight to warmth.
A bit of a weird one to me. I think the no liner thing is going to be a bad idea. I can see that thing filling up with pine needles and what not after some use.
My $0.02 - I like the kelvin active and wish they would have (1) added a hood or (2) done a straight vest with or without hood.