well there are two reasons they would be machine washable, one that they are made of some kind of superwash wool (merino or not) and that would make them questionable for covers, some people say superwash works great, some say it doesn't work at all. Most agree it is about = to acrylic or poly, not sure how that would work for pants.
The OTHER reason they may be machine washable is that they are already felted, if that is the case they will still work great no matter what, however felting fabric makes it a LOT less stretchy and supple, so their description makes me think it is more of a superwash thing then a pre-felted thing. And even after felting there will still be some shrinkage each washing for a while.
It is still a good price, but let us know if they work! (if you already ordered them) However if you haven't you might want to e-mail them and ask why they are machine washable if they are 100% wool. ~
Also they aren't really marketed to the cloth diapering use, so they are probably not intended for that, and there for they think washable is a good thing?