I asked the writer on Twitter and they responded, saying
The holy numbers are the relative lengths of the struts that is required in order for the Jansen mechanism to work (i.e. create motion).
Wikipedia calls this mechanism "Jansen's Linkage".
Theo Jansen himself calls these numbers "the eleven holy numbers". On his website, he describes how the leg system of the "strandbeest" was refined with the use of evolutionary computing.
Eleven holy numbers
Fifteen hundred legs with rods of random length were generated in the computer. It then assessed which of these approached the ideal walking curve. Out of the 1500, the computer selected the best 100. These were awarded the privilege of reproduction. Their rods were copied and combined into 1500 new legs. These 1500 new legs exhibited similarities with their parent legs and once again were assessed on their resemblance to the ideal curve. This process went through many generations during which the computer was on for weeks, months even, day and night. It finally resulted in eleven numbers denoting the ideal lengths of the required rods. The ultimate outcome of all this was the leg of Animaris Currens Vulgaris. This was the first beach animal to walk. And yet now and then Vulgaris was dead set against the idea of walking. A new computer evolution produced the legs of the generations that followed.
These, then, are the holy numbers: a = 38, b = 41.5, c = 39.3, d = 40.1, e = 55.8, f = 39.4, g = 36.7, h = 65.7, i = 49, j = 50, k = 61.9, l=7.8, m=15 . It is thanks to these numbers that the animals walk the way they do.
Note that this list actually contains 13 numbers; the last two are related to the crank driving the leg system rather than to the leg system itself.