This takes in the order of days, not years, without damaging the structural integrity by chemical decomposition that starts at relatively low temperatures.
If you are in a hurry, you can also put something like burnt lime in the chamber as that chemically captures the evaporating water without releasing it in these conditions. That way you can keep the humidity under 5% during the process if you spread the lime enough to capture the water as fast as it is driven out of the wood. You need to be careful to not burn the wood, but you should be able to use something from FLIR with some computer vision to check that no lumber overheats. The water also increases the absorption a lot, so this provides feedback that equalizes the humidity, instead of e.g. a runaway heating that would happen if dryer wood absorbs more microwaves.
And 13% moisture is relatively high. Kiln-dried hardwood for furniture is sold at about 8-10% moisture. Dried construction lumber is usually sold at about 19% moisture--just below the point which inhibits mold growth.[1] Heck, some of the construction-grade lumber at a big box store might already be at 13% just from sitting around for months inside the store. (Though they stack them poorly and this is one reason why the pieces are often so warped.)
I assume that they keep the pieces around for so long simply because they can. If your time horizon is decades, then you lose nothing by simply stacking the wood (without much stickering) and letting it sit. That doesn't take much effort, just patience. And considering how expensive and rare such long, defect-free pieces are, why risk losing pieces? I have no doubt they could dry the pieces faster, but they have better ways to spend their time. Their only immediate responsibility is to keep the years-long pipeline of wood filled.
[1] On the East Coast with a more humid climate, homebuilders purchase pre-dried lumber for framing. On the West Coast with a much dryer climate, builders traditionally buy wet wood for framing and the wood usually dries to <= 19% by the time they begin finishing the interior. (Wood primarily shrinks tangentially and radially, not longitudinally, so drying framing in place isn't usually an issue. See http://www.wood-database.com/wood-articles/dimensional-shrin...)