I agree with you, 20 yards would be a lot to devote to one quilt top. We don't really know for sure how much this pattern requires, since it hasn't told us. :(
That said, paper piecing does use more fabric than other patchwork. And pattern designers sometimes pad the fabric estimates so you don't run out. Most of us like to have leftovers for another project anyway.
It still seems like a lot, so I did some figuring... If you were to cover this quilt size ( 68x78 ) with 1" finished patches, and if you allowed .5 inch seam allowance instead of .25 inch (to allow for paper piecing), you would be using 2"x2" unfinished patches, and you would need over 16 yards of fabric. I am figuring that you would be able to use 40" of fabric width, but there's usually some waste there, as well as some waste when straightening the fabric for the first cut. Different sizes and shapes of patches make it really hard to figure, so this is just a simple example.
In addition, this is a complicated pattern, and I for one would have some mistakes that would result in more fabric not being usable. Then you would still need binding. Altogether I wouldn't be too surprized if the answer is really 20 yards.