Seattle moved from 8a to 8b when USDA put out their 2012 growing zone map, but the heat units are the lowest of basically any 8b in the world, so citrus is tough.
Well, that may be true compared to the US Southeast, but that is a little bit of an exaggeration to say the world.
There are some islands between Vancouver Island and the city of Vancouver (the Gulf Islands in the Salish Sea) that have lower heat units.
Even Whidbey Island, just a little north of Seattle has notoriously low heat units, definitely worse than Seattle. People there complain they need cold frames to be able to grow many types of vegetables.
The west coast of Scotland has some very low heat units, in zone 9. There are some parts of Iceland on the peninsulas that are surrounded by sea that are zone 8a with heat units that are so low it is nearly impossible to even grow an apple tree out in the open.
I'm about an hour south of Seattle, and the summers here actually get as hot as they do in coastal southern California, it's just that the time period of that heat is much shorter, maybe only lasts for a month and a half, and it's a much shorter growing season for citrus, maybe only a little more than a third of the year with temperatures high enough that the citrus can put on any significant growth, and I would say nearly half the year with the citrus in a state of complete dormancy.
To my understanding, much of the increased cold tollerence in mature trees comes from their larger mass. A larger tree simply holds more heat and has thicker bark than a small one.
That is what I've seen here (or actually closer to the Portland area, to be more precise) on a large Yuzu tree that looked like a hedge. The outer layer of leaves were almost fried, but the inner ones looked like they had not been affected as much.
I would guess that the outer leaves help block the movement of air. And also the water content in the outer leaves might release some heat energy as they freeze, helping to protect the inner part of the canopy.
That is why, to try to mimic that protective process, I cover a few of my more vulnerable varieties with a paper bag covering and put bottles of water inside, up against the plant. As little protection as this actually provides, I think it provides at least some, maybe 2 or 3 degrees of temperature difference.
Wind actually does make a significant difference. Citrus will freeze much faster in wind than with no wind. Plants usually maintain a temperature that is just a little bit warmer than the air surrounding them.
Even on the coldest nights here, most of the night is only slightly below the freezing point, but then it just dips down for three or four hours, and it will not dip below 15 °F for more than about half an hour (really rough approximation just for example). So I think protection from wind can be crucial.
The leaves of hardy citrus are not going to literally freeze if the air temperature is only just a little bit below the freezing point.
I don't know if this is specifically true of kumquat, however.